Clarify enforce_new_defaults help text
Clarify enforce_new_defaults help text

The help text isn't clear what happens when enforce_new_defaults is
False, which is the default behavior. Explicity call that out in the
help text since it's important for users to understand that behavior.

Change-Id: Iaed5682bc72f4c66adb9a40c6510b399314574df

openstack/oslo.policy