* Update all uses.

* apply policy per-course, even if multiple courses share course and org fields
* keep descriptors separate in xml store, so that if two such courses
  change the same module in different ways, it works.  Such edits will need to
  merged on CMS import...
* add get_instance(course_id, location) method to replace get_item(location).
  Update all the call sites
* tests, including a 2nd toy course with same course and org.

Acknowledge the fact that right now keystore is really just a module store. If we need a keystore that returns other objects, we can reexctract the base class into it's own module again

TODO:
  - support picking a different course name
  - make it use current module rendering path, get that working.

--HG--
branch : pmitros-mod-template