Use the read replica in the download data management command.

Second part of the fix for [ORA-629](https://openedx.atlassian.net/browse/ORA-629).
The companion PR in edx-submissions is https://github.com/edx/edx-submissions/pull/6  When that gets merged, I'll upload a new version of edx-submissions to PyPi and update the requirements file in ORA2.

@ormsbee Since Steve is out today, could I trouble you for a review?