Add ability to set keystone specific notification topic

PROD-32645

Change-Id: I661a24df78b119081292af730a7970bc834a096f
diff --git a/keystone/server/notification/messagingv2.yml b/keystone/server/notification/messagingv2.yml
index 2cc7e64..0bddb91 100644
--- a/keystone/server/notification/messagingv2.yml
+++ b/keystone/server/notification/messagingv2.yml
@@ -1,6 +1,8 @@
 parameters:
+  _param:
+    keystone_notification_topics: "${_param:openstack_notification_topics}"
   keystone:
     server:
       notification:
         driver: messagingv2
-        topics: "${_param:openstack_notification_topics}"
+        topics: "${_param:keystone_notification_topics}"