Fix test_provider_token_expiration_validation transient failure

The keystone.tests.test_token_provider.TestTokenProvider.
test_provider_token_expiration_validation test would fail if the
tests weren't run before the sample token expired. This could
happen if the system was slow and the previous tests took a long
time causing a potential transient failure.

Change-Id: Ia662fcdc0cd26568fa8ea9f27d0a34cb1bfe46e6
Closes-Bug: #1278550
Merge "Fix test_provider_token_expiration_validation transient failure"