Add ability to set keystone specific notification topic

PROD-32645

Change-Id: I661a24df78b119081292af730a7970bc834a096f
diff --git a/barbican/server/cluster.yml b/barbican/server/cluster.yml
index 110ff29..558f08b 100644
--- a/barbican/server/cluster.yml
+++ b/barbican/server/cluster.yml
@@ -36,6 +36,8 @@
           enabled: ${_param:barbican_memcache_security_enabled}
           strategy: ${_param:openstack_memcache_security_strategy}
           secret_key: ${_param:barbican_memcache_secret_key}
+      # Keystone notification topic. Should be overridden from cluster level (see PROD-32645)
+      ks_notifications_topic: notifications
   linux:
     system:
       package: