auto-migrate old mongo to draft modulestore
Move modulestore django settings migration code down into the modulestore layer so it works for all django-based callers.
reverting back to migrating the settings in the env files
Added deprecation warnings.
Merge pull request #4380 from edx/nimisha/mongo-bwc-config

auto-migrate old mongo to draft modulestore
Implement close_connections and drop_database on modulestores.