Modify export_to_xml to take an optional contentstore

In xmodule, export_to_xml only works with mongdb backed courses. However, for certain administrative tasks it could be useful to have a uniform interface to get copies of all the course types available to the LMS.

This PR makes the content store optional, and uses the module store to get the course in a way that works also for XML courses.

@cdodge @jtauber please review.