return enrollment object by id through api

fixes SIS-331

TestPlan:
 - create a new enrollment object in a course
 - get the enrollment id of the object
 - return the object through the api
   - /api/v1/accounts/:account_id/enrollments/:id
 - enrollment should return if proper privileges are present

Change-Id: Ib8c83af6d53e5d048047a024aa37228cb5eab8f2
Reviewed-on: https://gerrit.instructure.com/36772
Reviewed-by: Tyler Pickett <tpickett+gerrit@instructure.com>
Product-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
QA-Review: Tyler Pickett <tpickett+gerrit@instructure.com>
Tested-by: Tyler Pickett <tpickett+gerrit@instructure.com>