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/decision_engine/rpcapi.py

@@ -52,6 +52,8 @@ class DecisionEngineAPIManager(object):
 
     conductor_endpoints = []
     status_endpoints = [notification_handler.NotificationHandler]
+    notification_endpoints = []
+    notification_topics = []
 
     def __init__(self):
         self.publisher_id = CONF.watcher_decision_engine.publisher_id