auto-migrate old mongo to draft modulestore

@carsongee has pointed out that several members of the open edx community may be using the MongoModuleStore directly and could be affected by the new requirement to use the DraftModuleStore for importing a course.
 
"prior to this change parts of the Sysadmin Dashboard didn’t work properly with MixedModuleStore due to NotImplemented exceptions that are now implemented in that class, so we recommended to others that were interested in the feature to use just the mongo modulestore instead."

So this PR updates the migration code for the modulestore configuration settings to automatically migrate the use of the MongoModuleStore to the DraftModuleStore.  We are eventually (soon) merging these stores anyway.

@dmitchell @carsongee @dianakhuang  please review.
Would be best to let the application handle this, I think.