Replace deprecated [logger/LOG].warn with 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: I6b09f67bb63fbdf31903ec175db012fc50e87f16
Closes-Bug: 1508442
diff --git a/manila_tempest_tests/tests/api/base.py b/manila_tempest_tests/tests/api/base.py
index d57b493..b9763fb 100644
--- a/manila_tempest_tests/tests/api/base.py
+++ b/manila_tempest_tests/tests/api/base.py
@@ -613,8 +613,8 @@
                         client.delete_cgsnapshot(res_id)
                         client.wait_for_resource_deletion(cgsnapshot_id=res_id)
                     else:
-                        LOG.warn("Provided unsupported resource type for "
-                                 "cleanup '%s'. Skipping." % res["type"])
+                        LOG.warning("Provided unsupported resource type for "
+                                    "cleanup '%s'. Skipping." % res["type"])
                 res["deleted"] = True
 
     @classmethod