Add audit.strategy events

In this changeset, I implemented the following notifications:

- audit.strategy.start
- audit.strategy.end
- audit.strategy.error

Partially Implements: blueprint audit-versioned-notifications-api

Change-Id: I6ae8468caf8d215bc8bc694813beb4dc94f53fdb

watcher/objects/fields.py

@@ -126,7 +126,9 @@ class NotificationAction(BaseWatcherEnum):
     EXCEPTION = 'exception'
     DELETE = 'delete'
 
-    ALL = (CREATE, UPDATE, EXCEPTION, DELETE)
+    STRATEGY = 'strategy'
+
+    ALL = (CREATE, UPDATE, EXCEPTION, DELETE, STRATEGY)
 
 
 class NotificationPriorityField(BaseEnumField):