Fix DeprecationWarning for using logger.warn
Fix DeprecationWarning for using logger.warn

The logger.warn method is deprecated, use warning instead.

https://docs.python.org/3.6/library/logging.html#logging.warning

Change-Id: Id20ce8011aa0ac901a138132f7b1932c601e19ab
Story: 2006330
Task: 36070

openstack/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,openstack/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk