Devstack doesn't use gunicorn

[timmc-edx]

Unlike 2U's deployed environments, devstack uses Django's runserver command instead of gunicorn.

A/C:

• If feasible, switch devstack to use gunicorn
• If not, add docs on how to use gunicorn when desired, and why it's not the default

Notes:

• edx-platform's lms/docker_lms_gunicorn.py has a hardcoded workers = 17 which may not be appropriate for devstack. We may just be able to change it, depending on what actually uses this file these days.
• Can we still get autoreloading with gunicorn?