use log.warning instead of log.warn
use log.warning instead of log.warn

we noticed this in our py34 gate job:

oslo_log/versionutils.py:274
  DeprecationWarning The 'warn' method is deprecated, use 'warning' instead

Change-Id: I8a7d26b5ad55ff46cb8fcebf0b6812d4554ca2e9
Closes-Bug: 1508442

openstack/oslo.log