This will leverage the nose attrib plugin to allow us
to break tests apart on our build system to run
in parallel on separate nodes.

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

On devops recommendation, now handling the potential for an 'inconsistency
window' via a management command instead of a hacky "re-run the data migration"
bash script.

use 'commit_on_success' transaction decorator to avoid registering users in multiple cohort groups of a particular course
TNL-1331