Added notifications documentation page

In this changeset, I added a new Sphinx directive (from Nova) that
allows us to automatically display notification samples for of
all the upcoming notifications in Watcher.

Partially Implements: blueprint watcher-notifications-ovo

Change-Id: Id7d819ee21db6dc616c78faea483b883f77e3bd6

watcher/objects/notifications/exception.py

@@ -43,6 +43,7 @@ class ExceptionPayload(notificationbase.NotificationPayloadBase):
             exception_message=six.text_type(fault))
 
 
+@notificationbase.notification_sample('infra-optim-exception.json')
 @base.WatcherObjectRegistry.register_notification
 class ExceptionNotification(notificationbase.NotificationBase):
     # Version 1.0: Initial version