Add base support for cohorted group configurations

TNL-649

Introduced a new scheme field to user partitions that controls how a user gets assigned
to one of the partitions groups. The default scheme is random, but the next step will be
to introduce a new scheme that will assign the user based upon the cohort they belong to.
I think you may also need to override ne.

http://stackoverflow.com/questions/4352244/python-implementing-ne-operator-based-on-eq