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/
Would you expect such a method to compute the active children again, or have them passed in? If the latter, the method seems almost too trivial to be worth writing, and the former seems to be unnecessarily expensive. I would lean towards leaving it as is, especially since we now have tests that validate both pieces of logic. I could be convinced though...