Ensure service is restarted on unit file changes

At the moment we don't restart services if systemd unit file is changed.

We knowingly prevent systemd_service role handlers to execute
by providing `state: started` as otherwise service will be restarted twice.
With that now  we ensure that role handlers will also listen for systemd
unit changes.

Change-Id: I4a3346c90825a4bf0b416943286696fa529f526d

handlers/main.yml

@@ -20,3 +20,4 @@
   with_items: "{{ filtered_octavia_services }}"
   listen:
     - "venv changed"
+    - "systemd service changed"

vars/main.yml

@@ -29,7 +29,8 @@ filtered_octavia_services: |-
   {%     set _ = value.update(
            {
              'service_key': key,
-             'enabled': 'yes',
+             'enabled': value['enabled'] | default(True),
+             'state': value['state'] | default('started'),
              'config_overrides': value.init_config_overrides
            }
          )