The FakeLog defined in keystoneclient/tests/unit/test_http.py isn't
used anywhere. This patch removes it.
fixtures package already provides a FakeLogger for us to use,
so we really don't need to maintain a private implementation,
this patch removes FakeLog in test_auth_token_middleware and replaces
it with fixtures.FakeLogger
Change-Id: I6aaf761a9676edf5bd799d22b79497be1d423e7c