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.

systemd/README.rst

+Manage GWCelery processes with systemd
+======================================
+
+This directory provides support for starting, stopping, and supervising
+GWCelery using systemd_, the service manager for Linux. Systemd is present on
+many Linux systems 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.
+
+There is also an instance of systemd for each user that can manage that user's
+services. This example uses the per-user systemd instance. The following
+instructions were tested on Debian buster.
+
+1.  Check out this branch and install the code with pip as you normally would::
+
+        $ git clone https://git.ligo.org/leo-singer/gwcelery ~/src
+        $ cd ~/src
+        $ git checkout --track origin/systemd
+        $ pip3 install --user .
+
+    As usual, also make sure that you have provided authentication information
+    for GraceDb by running ``ligo-proxy-init`` and for LVAlert by completing
+    your ``~/.netrc`` file.
+
+2.  The command above installed the script ``gwcelery`` into ``~/.local/bin``.
+    Create an environment variable file to add it to the PATH for systemd::
+
+        $ mkdir -p ~/.config/environment.d
+        $ cd ~/.config/environment.d
+        $ echo 'PATH=$HOME/.local/bin:/usr/local/bin:/usr/bin:/bin' > 10-path.conf
+
+    If you want to use a non-default GWCelery configuration, then this is a
+    good time to add the ``CELERY_CONFIG_MODULE`` environment variable to a
+    file in ``~/.config/environment.d``.
+
+3.  Create a symbolic link to add the directory that contains this README file
+    to the user systemd unit search path::
+
+        $ mkdir -p ~/.local/share/systemd
+        $ cd ~/.local/share/systemd
+        $ ln -s ../../../src/systemd user
+
+4.  Tell systemd to reload configuration files::
+
+        $ systemctl --user daemon-reload
+
+5.  Tell systemd to bring up GWCelery::
+
+        $ systemctl --user start gwcelery.target
+
+6.  You can now manage GWCelery like any other system service. Here are some
+    example commands to try::
+
+        $ systemctl --user status gwcelery.target
+        $ systemctl --user stop gwcelery.target
+        $ systemctl --user restart gwcelery.target
+
+.. _systemd: https://freedesktop.org/wiki/Software/systemd/