Assert exceptions based on API, not string messages

The tests/sqlalchemy/test_exc_filters.py tests were asserting
the exact string format of SQLAlchemy exceptions.
SQLAlchemy is improving its exception format in version 1.0,
thereby breaking these tests.  Instead of supporting multiple
string formats, change the assertions to compare specific
fields per the oslo.db.exceptions and sqlalchemy.exc APIs.

Change-Id: Ie98ac3ca82a1052c463ca9ca5d41e228935497b3
Merge "Assert exceptions based on API, not string messages"