I agree that we need a consistent pattern for resetting `ContentStore` state.  However, I'm not sure that `ModuleStoreTestCase` is the right place to put this.  `contentstore` is in a different Django app, and many tests need only the modulestore flushed.  I'd like to avoid unnecessary Mongo operations if possible, since they're likely to slow down the test suite. 

In any event, Chris's changes don't apply to the contentstore settings.  Could we maybe create a backlog item for refactoring `ContentStore` tests, but say that it's outside the scope of this PR?