Retry on deadlock in instance_metadata_delete

Add the retry_on_deadlock decorator to the instance_metadata_delete
function in the sqlalchemy db API.  A tempest test failure shows that it
would benefit from it.

Change-Id: I39547fbbbabc310b78493df0dc055bc320995770
Closes-bug: #1226076
Merge "Retry on deadlock in instance_metadata_delete"