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/
We need to figure out what the API should be in XBlock for doing something like this. `modulestore` is currently an XModule/edx-platform specific api. It would be nice to have an XBlock api that other platforms can code against.