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/
@cpennington I was hoping for someplace in common so everyone could refer to those constants. How about x_module since that is where student_view and studio_view are defined?

Part of my confusion is that these methods just appear to be conventions-- they aren't defined in the xblock API itself.