bridge.o.o : install ansible 2.7.3

This installs Ansible 2.7.3 on bridge.o.o to incorporate fixes for [1]
which is currently stopping the cloud-launcher from running.

Currently every run it hits citycloud Lon1 and tries to delete it's
router

 TASK [cloud-launcher : Processing router openstackci-router1 for openstackci-citycloud Lon1] ***
 Monday 12 November 2018  04:07:48 +0000 (0:00:00.430)       0:07:45.811 *******
 fatal: [localhost]: FAILED! => {"changed": false, "msg": "Error
 detaching interface from router c7197a8f-096a-4488-a3ae-16fdce0ea580
 ...  cannot be deleted, as it is required by one or more floating
 IPs."}

Although it doesn't succeed, it's probably better that it isn't even
trying...

A prior version of this installed the unreleased stable branch to
bring this in, but didn't end up with enough reviews.  I've left
behind how to do that as a breadcrumb should we need to do similar in
the future (we do seem to have a nack of tickling Ansible bugs :)

[1] 951572bec1

Change-Id: I8f112ba994040c52c7b3c7ee6fd6f5a69fd22919

playbooks/bridge.yaml

@@ -5,9 +5,14 @@
     - pip3
     # Note for production use we expect to take the defaults; unit
     # test jobs override this to test with latest upstream ansible.
+    # For example, if there is a fix on the ansible stable branch we
+    # need that is unreleased, you could do the following:
+    #
+    #  install_ansible_name: '{{ bridge_ansible_name | default("git+https://github.com/ansible/ansible.git@stable-2.7") }}'
+    #  install_ansible_version: '{{ bridge_ansible_version | default(None) }}'
     - role: install-ansible
       install_ansible_name: '{{ bridge_ansible_name | default("ansible") }}'
-      install_ansible_version: '{{ bridge_ansible_version | default("2.7.0") }}'
+      install_ansible_version: '{{ bridge_ansible_version | default("2.7.3") }}'
       install_openstacksdk_name: '{{ bridge_openstacksdk_name | default("openstacksdk") }}'
       install_openstacksdk_version: '{{ bridge_openstacksdk_verison | default("latest") }}'
     - root-keys