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/common/utils.py

@@ -132,6 +132,10 @@ def get_cls_import_path(cls):
     return module + '.' + cls.__name__
 
 
+def strtime(at):
+    return at.strftime("%Y-%m-%dT%H:%M:%S.%f")
+
+
 # Default value feedback extension as jsonschema doesn't support it
 def extend_with_default(validator_class):
     validate_properties = validator_class.VALIDATORS["properties"]