Sync nova with oslo DB exception cleanup.

DB exceptions have moved to openstack/common/db/exception module so that
they can be shared with multiple DB implementations.

Deadlock checking was also added to oslo to consolidate with
DuplicateKey checking.  This allows us to clean up our
_retry_on_deadlock decorator in sqlalchemy/api.py

Fixes unrelated pep8 issue with duplicate test in test_compute also.

Change-Id: I7e985b384d1ef345e0d67c919b84b4faff869699
Merge "Sync nova with oslo DB exception cleanup."