Fix migration on older postgres

Since the move to oslo.db, sqlalchemy no longer raises
ProgrammingError.  Instead, it is wrapped in a
oslo.db.exception.DBError. This causes migration to fail on older
postgres, which relies on catching this error to use the correct
constraint name.

Change-Id: Icf7b20eb4e09f3c91aa3df3bcea497f810a6f907
Merge "Fix migration on older postgres"