Fix mock import in unit tests

We allowed the use of the third party mock lib, even though we no longer
declare it as a test-requirement. That should have failed, but there
must still be a dependency that pulls it in for us. But unit tests will
start failing once more dependencies move away from it, so we need to
clean that up.

Change-Id: I79d647e8c8a060eacfa5cbee39a99344adc0ae94
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

glance_store/tests/unit/common/test_fs_mount.py

@@ -12,10 +12,10 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
-import fixtures
-import mock
 import sys
+from unittest import mock
 
+import fixtures
 from oslo_concurrency import processutils
 from oslo_config import cfg
 from oslotest import base