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/
Ah, yes that's right. Strange that list1 + list2 works, but list1 - list2 does not.

Andy, you can use a list comprehension.
http://stackoverflow.com/questions/4211209/remove-all-the-elements-that-occur-in-1-list-from-another