Revert "Remove warning message when using old and new engine facade"

Partial revert. We made a mistake including this log message every
time this condition happens. Now the Neutron server debug logs are
polluted with this message.

This reverts commit 77b761dfe4.

Change-Id: I86366a151f132d59f250368e4e206509c1a45d3d

neutron_lib/context.py

@@ -15,18 +15,15 @@
 import collections
 import copy
 import datetime
+import warnings
 
 from oslo_context import context as oslo_context
 from oslo_db.sqlalchemy import enginefacade
-from oslo_log import log as logging
 
 from neutron_lib.db import api as db_api
 from neutron_lib.policy import _engine as policy_engine
 
 
-LOG = logging.getLogger(__name__)
-
-
 class ContextBase(oslo_context.RequestContext):
     """Security context and request information.
 
@@ -150,9 +147,11 @@ class Context(ContextBaseWithSession):
         # TODO(akamyshnikova): checking for session attribute won't be needed
         # when reader and writer will be used
         if hasattr(super(Context, self), 'session'):
-            LOG.debug('context.session is used with and without new '
-                      'enginefacade. Please update the code to use new '
-                      'enginefacede consistently.')
+            if self._session:
+                warnings.warn('context.session is used with and without '
+                              'new enginefacade. Please update the code to '
+                              'use new enginefacede consistently.',
+                              DeprecationWarning)
             return super(Context, self).session
         if self._session is None:
             self._session = db_api.get_writer_session()