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/
I rewrote the code to have a single ``active_and_inactive_children``` that returns a tuple as you suggested. It makes the code much cleaner as the callers always want both and, as you pointed out, my old ```inactive_children``` method was computing the active children again.