Failed to add implicit dependency with external resource

When validating new resource, we didn't avoid with adding implicit dependencies, some neutron/nova resource will fail if we happen to set external_id on other resources which it happens to check implicit dependency with.

For example if we try following template:

heat_template_version: queens

resources:
  router2:
    type: OS::Neutron::Router
    properties:
      external_gateway_info:
        network: 3fc38ae8-c5b8-451e-8430-7ea153e31af2

  ext_sub:
    type: OS::Neutron::Subnet
    external_id: 150514e8-228c-43de-b128-7f6e68b8e414

You will get following error log.
It failed because we try to find implicit dependencies during validate(and again during create/update), and the finding process try to use properties to validate the dependancy which external resource didn't provide. Therefore if failed.

DEBUG heat.engine.parameter_groups [None req-ddb54159-dc2d-4c3b-8778-8957de82b99c admin None] Validating Parameter Groups: OS::project_id, OS::stack_id {{(pid=7275) validate /opt/stack/heat/heat/engine/parameter_groups.py:42}}
ERROR oslo_messaging.rpc.server [None req-ddb54159-dc2d-4c3b-8778-8957de82b99c admin None] Exception during message handling: ValueError: Property network not assigned
ERROR oslo_messaging.rpc.server Traceback (most recent call last):
ERROR oslo_messaging.rpc.server File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/server.py", line 163, in _process_incoming
ERROR oslo_messaging.rpc.server res = self.dispatcher.dispatch(message)
ERROR oslo_messaging.rpc.server File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 220, in dispatch
ERROR oslo_messaging.rpc.server return self._do_dispatch(endpoint, method, ctxt, args)
ERROR oslo_messaging.rpc.server File "/usr/local/lib/python2.7/dist-packages/oslo_messaging/rpc/dispatcher.py", line 190, in _do_dispatch
ERROR oslo_messaging.rpc.server result = func(ctxt, **new_args)
ERROR oslo_messaging.rpc.server File "/usr/local/lib/python2.7/dist-packages/osprofiler/profiler.py", line 158, in wrapper
ERROR oslo_messaging.rpc.server result = f(*args, **kwargs)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/common/context.py", line 409, in wrapped
ERROR oslo_messaging.rpc.server return func(self, ctx, *args, **kwargs)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/service.py", line 793, in create_stack
ERROR oslo_messaging.rpc.server template_id)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/service.py", line 701, in _parse_template_and_validate_stack
ERROR oslo_messaging.rpc.server stack.validate()
ERROR oslo_messaging.rpc.server File "/usr/local/lib/python2.7/dist-packages/osprofiler/profiler.py", line 158, in wrapper
ERROR oslo_messaging.rpc.server result = f(*args, **kwargs)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/stack.py", line 872, in validate
ERROR oslo_messaging.rpc.server iter_rsc = self.dependencies
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/stack.py", line 429, in dependencies
ERROR oslo_messaging.rpc.server ignore_errors=self.id is not None)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/stack.py", line 515, in _add_implicit_dependencies
ERROR oslo_messaging.rpc.server res.add_dependencies(deps)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/resources/openstack/neutron/router.py", line 268, in add_dependencies
ERROR oslo_messaging.rpc.server subnet_net = res.properties.get(subnet.Subnet.NETWORK)
ERROR oslo_messaging.rpc.server File "/usr/lib/python2.7/_abcoll.py", line 382, in get
ERROR oslo_messaging.rpc.server return self[key]
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/properties.py", line 510, in __getitem__
ERROR oslo_messaging.rpc.server return self._get_property_value(key)
ERROR oslo_messaging.rpc.server File "/opt/stack/heat/heat/engine/properties.py", line 502, in _get_property_value
ERROR oslo_messaging.rpc.server raise ValueError(_('Property %s not assigned') % key)
ERROR oslo_messaging.rpc.server ValueError: Property network not assigned
ERROR oslo_messaging.rpc.server