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.
7 jobs for systemd in 2 minutes and 18 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Dist
passed #313753
dependencies/python3.6

00:00:16

passed #313752
sdist

00:00:24

 
  Test
passed #313754
bdist

00:00:24

passed #313757
doc

00:01:03

passed #313756
lint

00:00:27

passed #313755
test/python3.6

00:01:11

90.0%
 
  Deploy
passed #313758
docker/python3.6

00:00:42