Add Warning when we cannot notify

Calling Notifier.audit() won't work with LogDriver as the
logger does not have a audit level anymore, So let's at
least not fail silently and let the operators know that
we are dropping stuff on the floor.

Closes-Bug: #1518170
Change-Id: I74002c72e6763ea8b5df7d97d722619bd4a1950b

oslo_messaging/notify/_impl_log.py

@@ -16,6 +16,7 @@
 #    under the License.
 
 import logging
+import warnings
 
 from oslo_serialization import jsonutils
 from oslo_utils import strutils
@@ -40,3 +41,7 @@ class LogDriver(notifier.Driver):
         method = getattr(logger, priority.lower(), None)
         if method:
             method(strutils.mask_password(jsonutils.dumps(message)))
+        else:
+            warnings.warn('Unable to log message as notify cannot find a '
+                          'logger with the priority specified '
+                          '%s' % priority.lower())