WIP: Start, stop, and supervise gwcelery with systemd

Provide support for starting, stopping, and supervising GWCelery using systemd, the service manager for Linux. Systemd is present on many Linux systems including apparently all LDG cluster nodes and is responsible for bootstrapping userspace and bringing up system services.

The main advantage of using systemd instead of the existing htcondor technique is systemd can stop and restart the components of GWCelery in the proper sequence to minimize downtime and reduce the chances of losing in-flight tasks.

See the file systemd/README.rst included in this patch for instructions.