Refactor for testability of an upcoming change

confirm_token_not_expired() in keystoneclient/middleware/auth_token.py has
been moved out of the class to make it a function and be more testable.
Currently, there is no need to keep it within the class. An upcoming
commit makes fixes that rely on this refactor to be tested.

Change-Id: I8460a2ee663dec8be0f339735208779a3b988040
Merge "Refactor for testability of an upcoming change"