Fix datetime format in FakeCache

Since keystoneclient v0.4.0 auth_token middleware requires
the expiratin date and time of an authentication token to
be in ISO format.

This patch updates FakeCache which is used for unit tests
for ACL to generate expiration date and time according to
ISO 8601.

Closes-bug: #1238591
Change-Id: Ie6e542fe1acc7658ca62db5dfdfd949ff15c8373