db: Add @_retry_on_deadlock to service_update()

Add the _retry_on_deadlock decorator to the service_update() method of
nova's sqlalchemy API.  This decorator is scattered throughout the
sqlalchemy API for other methods that may encounter this error.  The
referenced bug report shows a trace from where it occurred on this
method.

Change-Id: I93b370d6457d2e85493be01a62a76404d228a6fa
Closes-bug: #1370191
Merge "db: Add @_retry_on_deadlock to service_update()"