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: I43e2269d4b1c425119c86919eb869941d621c6de
diff --git a/tox.ini b/tox.ini
index efdaacc..7e155cd 100644
--- a/tox.ini
+++ b/tox.ini
@@ -370,6 +370,7 @@
   T115 = checks:dont_put_admin_tests_on_nonadmin_path
   T116 = checks:unsupported_exception_attribute_PY3
   T117 = checks:negative_test_attribute_always_applied_to_negative_tests
+  T118 = checks:no_log_warn
 paths =
   ./tempest/hacking