Ability to change the associated group configuration for a split test

STUD-1658

My sandbox has the latest version of this PR.
http://studio.cahrens.m.sandbox.edx.org/
Not yet. @nedbat  and I were pretty focused on the LMS facing portions of XBlock, so we hadn't gotten to the lifecycle stuff (creation/deletion). I know that @dmitchell has been itching for more formalization of that.