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: I7e721580693a2d81b415a8ae0eb1963456d0195a

freezer/openstack/restore.py

@@ -286,7 +286,7 @@ class RestoreOs(object):
             except Exception as e:
                 LOG.exception(e)
                 if volume.status != "error":
-                    LOG.warn("Exception getting volume status")
+                    LOG.warning("Exception getting volume status")
 
         LOG.info("Deleting temporary image {}".format(image.id))
         self.client_manager.get_glance().images.delete(image.id)