Drop one more use of mock lib

We've switched over to unittest.mock and removed the third party mock
package from our test-requirements.txt file. We used to get it
indirectly through using oslotest, but that package has now dropped it
as well. This uncovered one case where we were still importing mock.

Change-Id: I88eca756bc142a74eb0d9aab36eeb22ff074824b
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

cinder/tests/unit/volume/drivers/test_nfs.py

@@ -1452,7 +1452,7 @@ class NfsDriverDoSetupTestCase(test.TestCase):
         super(NfsDriverDoSetupTestCase, self).setUp()
         self.context = mock.Mock()
         self.create_configuration()
-        self.override_config('compute_api_class', 'mock.Mock')
+        self.override_config('compute_api_class', 'unittest.mock.Mock')
 
     def create_configuration(self):
         config = conf.Configuration(None)