Add priority based queues to notifications.

Remove duplicate json encoding in notifier (rpc.cast does encoding... ) 
make no_op_notifier  match rabbit one for signature on notify()

nova/notifier/rabbit_notifier.py

@@ -13,7 +13,6 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import json
 
 import nova.context
 
@@ -28,10 +27,12 @@ flags.DEFINE_string('notification_topic', 'notifications',
 
 class RabbitNotifier(object):
     """Sends notifications to a specific RabbitMQ server and topic"""
-    pass
 
     def notify(self, payload):
         """Sends a notification to the RabbitMQ"""
         context = nova.context.get_admin_context()
-        topic = FLAGS.notification_topic
+        priority = payload.get('priority',
+                               FLAGS.default_notification_level)
+        priority = priority.lower()
+        topic = '%s.%s' % (FLAGS.notification_topic, priority)
         rpc.cast(context, topic, payload)