Python 3 deprecated the logger.warn method in favor of warning
Python 3 deprecated the logger.warn method in favor of 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: If47818a6ce8adb14bc275dc5a6b5ab4b9d189ff7
Closes-Bug: 1508442

openstack/python-magnumclient