Sahara: add oslo_messaging_notifications config

Sahara can send RPC notifications to Ceilometer as define here:
https://github.com/openstack/ceilometer/blob/master/ceilometer/pipeline/data/event_definitions.yaml#L147

oslo_messaging_notifications section MUST be managed in sahara.conf
file.

Change-Id: Ifdb5c1454a4940d4c1d8b521eb6c91529af142f3

ansible/roles/sahara/templates/sahara.conf.j2

@@ -42,6 +42,13 @@ project_domain_id = default
 user_domain_id = default
 auth_type = password
 
+[oslo_messaging_notifications]
+{% if enable_ceilometer | bool %}
+driver = messagingv2
+topics = 'notifications'
+{% else %}
+driver = noop
+{% endif %}
 
 [profiler]
 enabled = False