Remove use of deprecated LOG.warn

Remove use of deprecated LOG.warn

LOG.warn is deprecated and LOG.warning should be used.

This patch fixes up instances of LOG.warn usage and adds a
hacking check to make sure it doesn't creep back in.

See Logger.warning note here for background:
https://docs.python.org/3/library/logging.html

Also cleaned up some remaining instances where logging was
preformatting strings rather than passing in formatting
arguments to the logger to handle.

Change-Id: Id2e6cba489d8509601820b5aed83652f71be2bdc