TNL-160 Default cohorts, automatically created.

Please review:
@cahrens or @andy-armstrong 
@gwprice
Since you are touching these lines, can you change them to match our whitespace convention?
````
self.assertEquals(
    cohorts.get_cohort(user, course.id).id, 
    cohort.id,
   "user should be assigned to the right cohort"
)
````
same thing below