Thanks, Steve. I tried this but unfortunately it still wouldn't let me override the name of the network it's trying to create. Perhaps it's hard coded somewhere?

[stack@undercloud templates]$ heat stack-show 8d7d3fa7-ae27-4361-a271-8468f07354b3
WARNING (shell) "heat stack-show" is deprecated, please use "openstack stack show" instead
+-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Property              | Value                                                                                                                                                                       |
+-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| capabilities          | []                                                                                                                                                                          |
| creation_time         | 2016-10-12T12:52:08Z                                                                                                                                                        |
| deletion_time         | None                                                                                                                                                                        |
| description           | External network. Public traffic, Neutron l3router for                                                                                                                      |
|                       | floating IPs/SNAT, etc.                                                                                                                                                     |
| disable_rollback      | True                                                                                                                                                                        |
| id                    | 8d7d3fa7-ae27-4361-a271-8468f07354b3                                                                                                                                        |
| links                 | http://172.16.0.1:8004/v1/8d35712bab764e4cb8666dcd0c9a6a50/stacks/region-two-Networks-gp55udj5jo3g-ExternalNetwork-o6mko4qco7i5/8d7d3fa7-ae27-4361-a271-8468f07354b3 (self) |
| notification_topics   | []                                                                                                                                                                          |
| outputs               | [                                                                                                                                                                           |
|                       |   {                                                                                                                                                                         |
|                       |     "output_value": null,                                                                                                                                                   |
|                       |     "output_key": "OS::stack_id",                                                                                                                                           |
|                       |     "description": "Neutron external network"                                                                                                                               |
|                       |   }                                                                                                                                                                         |
|                       | ]                                                                                                                                                                           |
| parameters            | {                                                                                                                                                                           |
|                       |   "OS::project_id": "8d35712bab764e4cb8666dcd0c9a6a50",                                                                                                                     |
|                       |   "ExternalNetName": "external2",                                                                                                                                           |
|                       |   "OS::stack_id": "8d7d3fa7-ae27-4361-a271-8468f07354b3",                                                                                                                   |
|                       |   "OS::stack_name": "region-two-Networks-gp55udj5jo3g-ExternalNetwork-o6mko4qco7i5",                                                                                        |
|                       |   "ExternalNetAdminStateUp": "False",                                                                                                                                       |
|                       |   "ExternalAllocationPools": "[{u'start': u'192.168.122.130', u'end': u'192.168.122.159'}]",                                                                                |
|                       |   "ExternalNetValueSpecs": "{u'provider:physical_network': u'external', u'provider:network_type': u'flat'}",                                                                |
|                       |   "ExternalNetShared": "False",                                                                                                                                             |
|                       |   "ExternalInterfaceDefaultRoute": "192.168.122.1",                                                                                                                         |
|                       |   "ExternalNetCidr": "192.168.122.0/24",                                                                                                                                    |
|                       |   "ExternalSubnetName": "external_subnet",                                                                                                                                  |
|                       |   "ExternalNetEnableDHCP": "False"                                                                                                                                          |
|                       | }                                                                                                                                                                           |
| parent                | 04f8259f-8340-48f5-90de-411063ce4b23                                                                                                                                        |
| stack_name            | region-two-Networks-gp55udj5jo3g-ExternalNetwork-o6mko4qco7i5                                                                                                               |
| stack_owner           | admin                                                                                                                                                                       |
| stack_status          | CREATE_FAILED                                                                                                                                                               |
| stack_status_reason   | Resource CREATE failed: Conflict:                                                                                                                                           |
|                       | resources.ExternalNetwork: Unable to create the flat                                                                                                                        |
|                       | network. Physical network external is in use. Neutron                                                                                                                       |
|                       | server returns request_ids: ['req-6924165a-                                                                                                                                 |
|                       | 0f05-4667-8b96-b5604b14d0f1']                                                                                                                                               |
| stack_user_project_id | 1a92b336ce2c4713aba6149556e61438                                                                                                                                            |
| tags                  | null                                                                                                                                                                        |
| template_description  | External network. Public traffic, Neutron l3router for                                                                                                                      |
|                       | floating IPs/SNAT, etc.                                                                                                                                                     |
| timeout_mins          | 240                                                                                                                                                                         |
| updated_time          | None                                                                                                                                                                        |
+-----------------------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------+