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.
10 jobs for systemd in 5 minutes (queued for 2 seconds)
latest
Status Job ID Name Coverage
  Dist
passed #345750
dependencies/python3.6

00:03:09

passed #345751
dependencies/python3.7

00:03:07

passed #345749
sdist

00:00:35

 
  Test
passed #345752
bdist

00:00:25

passed #345756
doc

00:01:14

passed #345755
lint

00:00:29

passed #345753
test/python3.6

00:01:17

87.0%
passed #345754
test/python3.7

00:01:15

87.0%
 
  Deploy
passed #345757
docker/python3.6

00:00:32

passed #345758
docker/python3.7

00:00:31