Use unittest.mock instead of third party mock

Now that we no longer support py27, we can use the standard library
unittest.mock module instead of the third party mock lib.

Change-Id: I6ad6214a598760807572ab98ed9c359e7afdbc2f
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

searchlight_ui/tests/api_tests/test_searchlight_rest.py

@@ -12,7 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import mock
+from unittest import mock
+
 from openstack_dashboard.test import helpers as test
 
 from searchlight_ui.api.rest import searchlight