Support systemd as a process manager

[natefoo]

Also, prefer "passing through" galaxyctl exec so that the service units (or supervisor configs, when using supervisor) don't have to be rewritten every time there is a change. Regardless of what PM you use, you can run galaxyctl exec <service_name> to just run one of the services in the foreground, and this is now the command written to the process manager configs. If you don't want Gravity to write your systemd/supervisor configs, you can just write your own that call galaxyctl exec ... as necessary. This makes us behave more like a "normal" project that just deals with execution, not deployment, in a command line tool.

This change also pretty much removes the need for configstate. We should be able to remove it in the default case of a single config/instance of galaxy, e.g. if you set $GALAXY_CONFIG_FILE or a (new, not yet created) global -c/--galaxy-config option then we can just skip the configstate entirely.

I have to remember how to update the docs, and create a corresponding PR in Galaxy to update the docs and document how to set start_timeout and stop_timeout on handler defs.