Change LOG.warn to LOG.warning

Python 3 deprecated the logger.warn method, see:
https://docs.python.org/3/library/logging.html#logging.warning
so we prefer to use warning to avoid DeprecationWarning.

Change-Id: I3a057080b7b888b2c246ad9910ddba564b07dbd9
Closes-Bug: #1530742
diff --git a/tempest/api/compute/base.py b/tempest/api/compute/base.py
index aa8ee3f..eaef598 100644
--- a/tempest/api/compute/base.py
+++ b/tempest/api/compute/base.py
@@ -278,8 +278,8 @@
             # into the delete_volume method as a convenience to the caller.
             volumes_client.wait_for_resource_deletion(volume_id)
         except lib_exc.NotFound:
-            LOG.warn("Unable to delete volume '%s' since it was not found. "
-                     "Maybe it was already deleted?" % volume_id)
+            LOG.warning("Unable to delete volume '%s' since it was not found. "
+                        "Maybe it was already deleted?" % volume_id)
 
     @classmethod
     def prepare_instance_network(cls):