Add @_retry_on_deadlock to _instance_update()

This patch adds the @_retry_on_deadlock decorator
to _instance_update(), which has been observed to be
involved in some deadlock scenarios.  As it is the point
of transaction demarcation based on the presence of
get_session(), this is an appropriate point at which the
transaction can be re-attempted in the case of deadlock.

Change-Id: Id2cfdc129ea3cfc787b73f1f3e9aa286a7c0229a
Closes-bug: 1375467
Cherry-picked from: 68ed3c034ba1b0767694026000723e5255d24a64
Merge "Add @_retry_on_deadlock to _instance_update()" into stable/juno