Updated openstack/openstack
Project: openstack/oslo.db  50b8d1e8a1fbad5fa8f423229b993fb088282a15

Raise DBReferenceError on foreign key violation

SQLAlchemy session code now parses foreign key violations and returns a
new custom exception so we can track them.

Change-Id: If5610b5743203a5c34ac7df240e5332c39911c4a