Cleanup superfluous string comprehension and coersion

Remove the superfluous string comprehension and coersion to string
type in a join within the keystone.notifications module. The elements
in the namedtuple are already string types.

Change-Id: I107102fabc2f6671e10bcf1b5db40529fdb19d69

keystone/notifications.py

@@ -164,8 +164,7 @@ def register_event_callback(event, resource_type, callbacks):
     if event not in ACTIONS:
         raise ValueError(_('%(event)s is not a valid notification event, must '
                            'be one of: %(actions)s') %
-                         {'event': event,
-                          'actions': ', '.join(str(a) for a in ACTIONS)})
+                         {'event': event, 'actions': ', '.join(ACTIONS)})
 
     if not hasattr(callbacks, '__iter__'):
         callbacks = [callbacks]