Ensure doesn't use LOG.warn

This patchs ensure doesn't use LOG.warn because LOG.warn deprecated
in Python 3 [1]. Using LOG.warning should be used instead.

[1] https://docs.python.org/3/library/logging.html#logging.warning

TrivialFix

Change-Id: I464314b33b8d4f31b6be0a765019e1f5668cc6d9