Add unit test for fernet provider

There is some unit testing specific to the Fernet token provider that lives
outside of the test_v3_* and test_v2 test modules. This change just adds an
explict unit test to assert that the Fernet token providers doesn't require
persistent storage for tokens.

Closes-Bug: 1471967
Change-Id: Iad9c4acc1241ec56222759998a86d0abeae143dc
(cherry picked from commit 7d6e2a2805)

keystone/tests/unit/token/test_fernet_provider.py

@@ -42,6 +42,9 @@ class TestFernetTokenProvider(tests.TestCase):
         self.assertRaises(exception.NotImplemented,
                           self.provider._get_token_id, token_data)
 
+    def test_needs_persistence_returns_false(self):
+        self.assertFalse(self.provider.needs_persistence())
+
     def test_invalid_v3_token_raises_401(self):
         self.assertRaises(
             exception.Unauthorized,