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 expiration period passed. This could
happen if the system was slow and the previous tests took a long
time causing a potential transient failure.

Change-Id: I123aadd55770002fe1c9ed9b17669d5212198aa3
Closes-Bug: #1278550
(cherry picked from commit 170faf1468647e9a8246e6f7f157c7e5fd475ed5)