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: I6cdd4c35a52a014ba3c4dfe4cc2bd4d670c96bc3
Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>

watcher/tests/decision_engine/messaging/test_data_model_endpoint.py

@@ -13,7 +13,6 @@
 # implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
 import unittest
 from unittest import mock