Retry db.api.instance_destroy on deadlock

Update the instance_destroy method of the sqlalchemy db API to retry if
it fails because of a deadlock. The decorator that handles this was
introduced in I0293c62d2dd5ac036445bc639cabbd05ba016e83.

This patch does not include a unit test for this bug, as deadlocks are
very hard to reproduce.

Change-Id: I570d91e892a6e1ff4b3cdb8cf81acc483649835e
Closes-Bug: #1342086
Merge "Retry db.api.instance_destroy on deadlock"