Single service unit using the multprocessing process manager

[natefoo]

As @mvdbeek mentioned in galaxyproject/galaxy#20520, the multiprocessing process manager is much nicer to deal with for the simple foreground case. I am tempted to add a systemd unit that just runs it that way in a single service.

Upsides:

• Single service, much simpler for admins not deeply familiar with systemd
• No state or process manager (supervisor, systemd) config files need to be managed by Gravity

Downsides:

• No journald filtering by process, the journal is going to be very noisy
• No fault tolerance of individual processes, if something dies you have to notice and restart the entire service (all processes)
• No graceful restarts

For the last 2 points, it might be feasible to have the multiprocessing PM restart individual processes when they exit. It might also be feasible to have galaxyctl restart <service> find the right process and SIGTERM it. galaxyctl graceful could use the same process to terminate the gunicorns and do health checks the way it does for other PMs.