Ignore oslo.db deprecating sqlalchemy-migrate warning

In I59335b4f318bae2e29ab139cdea089a4d6e14305, oslo.db
is now emitting deprecation warnings for SQLAlchemy-migrate
functions.  This breaks Keystone tests which raise on
DeprecationWarning, so add to the filters.

Change-Id: I42f0abc2ddf8c53239d5098d5f32b667314b942d

keystone/tests/unit/core.py

@@ -681,6 +681,9 @@ class BaseTestCase(testtools.TestCase):
 
         warnings.filterwarnings('error', category=DeprecationWarning,
                                 module='^keystone\\.')
+        warnings.filterwarnings(
+            'ignore', category=DeprecationWarning,
+            message=r"Using function/method 'db_version\(\)' is deprecated")
         warnings.simplefilter('error', exc.SAWarning)
         if hasattr(exc, "RemovedIn20Warning"):
             warnings.simplefilter('ignore', exc.RemovedIn20Warning)