Change LOG.warn to LOG.warning
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: I3b8651ee69c9c817ad47db13b06f2f92077a3923
Closes-Bug: #1530742

openstack/kingbird,openstack/kingbird