fix(controller): restart celery or gunicorn in reload script

Previously the /app/bin/reload script invoked by confd assumed that
pidfiles for celery and gunicorn always exist, which is not true in
some failure cases. This only sends them a SIGHUP if they are running,
and restarts them otherwise.

Fixes #1100.

TESTING: Rebuild and restart the controller, then `nsenter` deis-controller and try killing either process set and running /app/bin/reload, restarting deis-builder and other containers, and seeing that the controller actually reloads and that celery and gunicorn process continue to run.