Integrate with opaque_keys package for checking course_ids.

Also change to running Python 2.7 on clusters, since opaque keys
requires that.

* Export opaque keys and dependencies to worker instances in Hadoop.
* Support for checking if course_id is valid.
* Rudimentary support for getting course_id from URL.
* Make sure filenames from course_ids are legal, replacing characters as
  needed.
* Identify organization for each event as close to original algorithm as
  possible on existing data, but working for opaque keys as well.

Acked-by: Brian Wilson <brian@edx.org>