Fix LOG.warn to LOG.warning

logging.warn is deprecated in Python 3.
https://docs.python.org/3/library/logging.html#logging.warning

Change-Id: I5ba02445e42ff1dae55ac59d0da2b0bd67fea495
diff --git a/tempest/clients.py b/tempest/clients.py
index b13b809..edc34bd 100644
--- a/tempest/clients.py
+++ b/tempest/clients.py
@@ -117,7 +117,7 @@
                     configuration[service_for_config] = (
                         config.service_client_config(service_for_config))
             except lib_exc.UnknownServiceClient:
-                LOG.warn(
+                LOG.warning(
                     'Could not load configuration for service %s' % service)
 
         return configuration