Disable discussion service setting for lettuce tests

@nasthagiri 

Enabling the discussion service integration has other implications - every time a user is created or logs into lms or studio it tries and fails to update a corresponding user record on the cs_comments_service side. This is causing the tests to use extra resources, and clogging up the logs (which is causing confusion for people troubleshooting actual errors). 

I'd like to turn it off and make sure it stays off for lettuce tests.