Enhancement mocking config enable only

- Updates MongoDB and Cassandra to enable/disable the backend mocking via a config variable (is_mocking) under the testing section of their respective configuration section. The deuce/tests/__init__.py will then load the correct mocking backend if their appropriate is_mocking is set to True.

Note: mocking does not apply to the sqlite driver.

Depends on PR #84
Closes CBDEUCE-155