Moved xml_store_config, mongo_store_config, and draft_mongo_store_config...

Moved xml_store_config, mongo_store_config, and draft_mongo_store_config to xmodule/modulestore/tests/django_util.py
Removed some repeated instances of xml_store_config.
Reverted some changes to mongo_store_config so each modulestore gets it's own uuid.

@JonahStanley @wedaly @jzoldak qq