Split/paver LMS-11211

This PR updates the Paver code for Lettuce Acceptance and Bokchoy tests so we can test them with different modulestores (namely, Split and Draft).

- *Acceptance Tests:*  By default, they will continue to be tested with the Draft Modulestore.  To test with Split, you would need to call paver with --default_store='split'.  Unfortunately, many tests are currently failing with Split and would need to be diagnosed and fixed.  We are not undertaking that work at this time.

- *BokChoy Tests:* By default, Bokchoy tests will now be run with the Split Modulestore.  As part of this story, we had taken the time to fix all the tests that failed with Split.

@jzoldak @cpennington  Have reviewed the commits that changed Paver.
@dmitchell Please review the remaining commits.
@cahrens Please review the last commit related to Studio's drag and drop feature.
Ah, makes sense. In that case, I don't think you need to specify `None`, since that's the default default-value for `.get`.