In keystonemiddleware 2.0.0 the token cache keys are sha256 hashes of the
token key. For versions of keystonemiddleware < 2.0.0 this is not the
case, so this patch is changing the FakeMemcache class to support both.
Change-Id: I14f6a47437ca861d3135bedbfe2b3a54de211182
Closes-Bug: #1466405
(cherry picked from commit 13bcebcaf3)