Segregate the mongo db config from the modulestore config

as a first step to separating db connection from modeling
and to support location mapper and contentstore using same db config as modulestore

@e0d please review
@chrisndodge @cpennington please review. Ed wanted to complete my separation of the db config from the modulestore config; so, rather than having the modulestore config as a subset of the db config, I separated them
and pass the db config as a dict instead of kwargs to modulestore inits.