Make course_id a required arg to split.create_course

and remove code which made it unique. Instead raise exception on dupe.
STUD-1359

I added a bunch of calls to dropping the dbs and closing connections too. (was getting duplicate course errors as db contents bled over from one test to another)

@singingwolfboy @andy-armstrong Please review. This builds on and will conflict w/ https://github.com/edx/edx-platform/pull/2842 (conflict b/c it touches same lines)