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: Ifb8e1272f8d954250d141cfa09d08bbc554de994
diff --git a/designate_tempest_plugin/data_utils.py b/designate_tempest_plugin/data_utils.py
index c9d3f6d..23e93a6 100644
--- a/designate_tempest_plugin/data_utils.py
+++ b/designate_tempest_plugin/data_utils.py
@@ -96,8 +96,8 @@
         quotas_dict['api_export_size'] = \
             api_export_size or data_utils.rand_int_id(100, 999999)
     else:
-        LOG.warn("Leaving `api_export_size` out of quota data due to: "
-                 "https://bugs.launchpad.net/designate/+bug/1573141")
+        LOG.warning("Leaving `api_export_size` out of quota data due to: "
+                    "https://bugs.launchpad.net/designate/+bug/1573141")
 
     return quotas_dict