Using LOG.warning replace LOG.warn

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.

Closes-Bugs: #1529913

Change-Id: Icc01ce5fbd10880440cf75a2e0833394783464a0
Co-Authored-By: Gary Kotton <gkotton@vmware.com>

HACKING.rst

@@ -23,6 +23,8 @@ Neutron Specific Commandments
 - [N331] Detect wrong usage with assertTrue(isinstance()).
 - [N332] Use assertEqual(expected_http_code, observed_http_code) instead of
          assertEqual(observed_http_code, expected_http_code).
+- [N333] Validate that LOG.warning is used instead of LOG.warn. The latter
+  is deprecated.
 
 Creating Unit Tests
 -------------------