Use LOG.warning instead of deprecated LOG.warn

The LOG.warn method is deprecated[1] and the LOG.warning method should
be used instead.

[1] https://docs.python.org/3/library/logging.html#logging.warning

Change-Id: I5211a236492910d759f3c4beb04d9ffa156007ae
diff --git a/heat_tempest_plugin/tests/api/test_heat_api.py b/heat_tempest_plugin/tests/api/test_heat_api.py
index d256437..9720fe3 100644
--- a/heat_tempest_plugin/tests/api/test_heat_api.py
+++ b/heat_tempest_plugin/tests/api/test_heat_api.py
@@ -63,8 +63,8 @@
                 keystoneauth1.exceptions.discovery.DiscoveryFailure,
                 keystoneauth1.exceptions.connection.UnknownConnectionError,
                 keystoneauth1.exceptions.connection.ConnectFailure):
-            LOG.warn("Keystone auth exception: %s: %s" % (sys.exc_info()[0],
-                                                          sys.exc_info()[1]))
+            LOG.warning("Keystone auth exception: %s: %s" %
+                        (sys.exc_info()[0], sys.exc_info()[1]))
             # Clear the auth_url, as there is no point in tempest trying
             # to authenticate later with mis-configured or unreachable endpoint
             conf.auth_url = None