kolla-ansible bootstrap-servers fails when api_interface is invalid

When bootstrapping a host for kolla-ansible the 'kolla-ansible bootstrap-servers' command can fail if for any of the hosts the 'api_interface' is invalid. This could be true for a number of reasons:

- The 'api_interface' variable is undefined.
- The host does not have a network interface matching 'api_interface'.

Valid causes for the second point include:
- The Ansible control host (localhost) is in the inventory but is not intended to be a target host.
- One of the hosts (currently must be localhost) is a Bifrost host, which does not need a valid api_interface.
- The services mapped to the host do not require the use of an api_interface.

Steps to reproduce:

Install kolla-ansible
Configure /etc/kolla/globals.yml setting api_interface to an invalid value or empty.
In the Ansible inventory file, ensure that all required hosts are in the baremetal group.
Run 'kolla-ansible bootstrap-servers'

Expected results:
Kolla-ansible bootstraps all hosts in the baremetal group.

Actual results:
The command fails with the following output:

[vagrant@localhost ~]$ kolla-ansible bootstrap-servers
Bootstraping servers : ansible-playbook -i /usr/share/kolla-ansible/ansible/inventory/all-in-one -e @/etc/kolla/globals.yml -e @/etc/kolla/passwords.yml -e CONFIG_DIR=/etc/kolla -e action=bootstrap-servers /usr/share/kolla-ansible/ansible/kolla-host.yml

PLAY [Apply role baremetal] ****************************************************

TASK [baremetal : include] *****************************************************
included: /usr/share/kolla-ansible/ansible/roles/baremetal/tasks/bootstrap-servers.yml for localhost

TASK [baremetal : include] *****************************************************
included: /usr/share/kolla-ansible/ansible/roles/baremetal/tasks/pre-install.yml for localhost

TASK [baremetal : Install python2 and python-simplejson] ***********************
changed: [localhost]

TASK [baremetal : Gather facts] ************************************************
ok: [localhost]

TASK [baremetal : Ensure localhost in /etc/hosts] ******************************
ok: [localhost]

TASK [baremetal : Generate /etc/hosts for all of the nodes] ********************
fatal: [localhost]: FAILED! => {"failed": true, "msg": "Unexpected templating type error occurred on ({% for host in groups['all'] %}\n{{ hostvars[host]['ansible_' + hostvars[host]['api_interface']]['ipv4']['address'] }} {{ hostvars[host]['ansible_hostname'] }}\n{% endfor %}\n): cannot concatenate 'str' and 'NoneType' objects"}
 [WARNING]: Could not create retry file '/usr/share/kolla-ansible/ansible/kolla-host.retry'. [Errno 13] Permission denied: u'/usr/share/kolla-ansible/ansible/kolla-host.retry'

PLAY RECAP *********************************************************************
localhost : ok=5 changed=1 unreachable=0 failed=1

Command failed ansible-playbook -i /usr/share/kolla-ansible/ansible/inventory/all-in-one -e @/etc/kolla/globals.yml -e @/etc/kolla/passwords.yml -e CONFIG_DIR=/etc/kolla -e action=bootstrap-servers /usr/share/kolla-ansible/ansible/kolla-host.yml