Make monasca notification templates optional

backport: stein

If I deploy monasca by setting enable_monasca to true, the monasca_notification
restarts with the following error:

ERROR:__main__:MissingRequiredSource: /var/lib/kolla/config_files/notification_templates/* file is not found

These templates are optional, so we need to mark this directory as optional in
config.json.

Change-Id: Ia2dd835daa7ab1153617cc92f17c2d8d498c73e0
Closes-Bug: #1823726

ansible/roles/monasca/templates/monasca-notification/monasca-notification.json.j2

@@ -11,7 +11,8 @@
             "source": "{{ container_config_directory }}/notification_templates/*",
             "dest": "/etc/monasca/",
             "owner": "monasca",
-            "perm": "0600"
+            "perm": "0600",
+            "optional": true
         }
     ],
     "permissions": [