Notification and CDM partial update

In this changeset, I implemented the notification handling (Rx only)
system for consuming incoming notifications, more especially the Nova
ones. The notifications handlers also contain the logic which
incrementally updates the Compute model.

Change-Id: Ia036a5a2be6caa64b7f180de38821b57c624300c
Partially-implements: blueprint cluster-model-objects-wrapper

watcher/tests/decision_engine/audit/test_audit_handlers.py

@@ -73,9 +73,9 @@ class TestOneShotAuditHandler(base.DbTestCase):
             'audit_uuid': self.audit.uuid})
 
         calls = [call_on_going, call_succeeded]
-        messaging.status_topic_handler.publish_event.assert_has_calls(calls)
+        messaging.publish_status_event.assert_has_calls(calls)
         self.assertEqual(
-            2, messaging.status_topic_handler.publish_event.call_count)
+            2, messaging.publish_status_event.call_count)
 
 
 class TestContinuousAuditHandler(base.DbTestCase):