Enable overriding the service setup host python interpreter

In order to enable the service setup host python interpreter to
be changed easily, we make it a variable. This will be useful
when someone sets the service setup host to be the utility
container, because we'll be able to set this var by default.

Change-Id: I25e6d1a047423b719a5f77bb46b529a667dbfa7a

defaults/main.yml

@@ -20,6 +20,7 @@ debug: False
 # for the service setup. The host must already have
 # clouds.yaml properly configured.
 watcher_service_setup_host: "{{ openstack_service_setup_host | default('localhost') }}"
+watcher_service_setup_host_python_interpreter: "{{ openstack_service_setup_host_python_interpreter | default((watcher_service_setup_host == 'localhost') | ternary(ansible_playbook_python, ansible_python['executable'])) }}"
 
 # Set the package install state for distribution and pip packages
 # Options are 'present' and 'latest'

tasks/watcher_service_setup.yml

@@ -21,8 +21,7 @@
 - name: Setup the service
   delegate_to: "{{ watcher_service_setup_host }}"
   vars:
-    ansible_python_interpreter: >-
-      {{ (watcher_service_setup_host == 'localhost') | ternary(ansible_playbook_python, ansible_python['executable']) }}
+    ansible_python_interpreter: "{{ watcher_service_setup_host_python_interpreter }}"
   block:
     - name: Add service to the keystone service catalog
       os_keystone_service: