Refactored Watcher objects to use OVO

In this changeset, I modified all existing Watcher objects to now
rely on oslo.versionedobjects as a base.

Change-Id: I3c9b1ca6da529d128743b99020350f28926ea1a2
Partially-Implements: blueprint watcher-versioned-objects

requirements.txt

@@ -21,6 +21,7 @@ oslo.reports>=0.6.0 # Apache-2.0
 oslo.serialization>=1.10.0 # Apache-2.0
 oslo.service>=1.10.0 # Apache-2.0
 oslo.utils>=3.16.0 # Apache-2.0
+oslo.versionedobjects>=1.13.0 # Apache-2.0
 PasteDeploy>=1.5.0 # MIT
 pbr>=1.6 # Apache-2.0
 pecan!=1.0.2,!=1.0.3,!=1.0.4,!=1.2,>=1.0.0 # BSD