Retry on deadlock in instance_metadata_update

THe instance_metadata_update() method in the sqlalchemy db API may hit a
DBDeadlock, as shown in the bug report.  Apply the necessary decorator
that will have the method retry in that case.

Change-Id: Ice3f13857ba8f4ee1d1d2dc06cef293e6d17daca
Closes-bug: #1262154
Merge "Retry on deadlock in instance_metadata_update"