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.
Can we use http://stevedore.readthedocs.org/en/latest/ for this plugin mechanism? (I'd like to standardize on that as the backing mechanism for all of our python plugins across the platform).