Celery tweaks to make periodic tasks run on own queue separate from on demand queue

other goodies
 * remove celerymon, use celery-flower
 * get rid of gevent concurrency and just use good ol' processes
 * have celery-flower run off management command
 * use new celery manage.py syntax
 * 2 celery workers on one machine (for now) - one for consuming celery, other for consuming the new periodic queue

related prs:
https://github.com/dimagi/ctable/pull/31
https://github.com/dimagi/payments/pull/26
https://github.com/dimagi/django-soil/pull/9