Fix auth logic for PUT /v2/alarms

The existing auth logic breaks threshold evaluation in a couple
of ways:

* arbitrary alarms are visible to the administrative ceilometer
  user, but state transformations cannot be recorded unless the
  alarm is actually owned by this user also

* using the passed X-User-Id header to set the user ID on the
  updated alarm would cause ownership of all alarms to be eventually
  assumed by the ceilometer user (once a state transformation
  occurs)

Change-Id: I0fcef188741c59fe2691e2009c28182dab51eb0b
Merge "Fix auth logic for PUT /v2/alarms"