Until now, standalone subsystem was started as a daemon first time a terminal
was openned and since it was running as daemon, stopping the theia instance
did not stop any of the services running in the embedded slapos.

Before nexedi/slapos.core!265 there was
two supervisor running as daemon:
 - ~/srv/slapos/etc/supervisord.conf which runs slapos proxy etc
 - ~/srv/slapos/inst/etc/supervisord.conf with runs instances in the embedded
   slapos
After, the second one runs as a service in the first one, but the first one
was still running as daemon.

This changes so that the first supervisor runs as a service managed by the
Theia instance, so stopping Theia instance will effectively stop the services.

When upgrading, running instances should continue to run as detached. To
attach them to the new service, procedure could be something like this

Stop supervisors inside Theia instance:

  supervisorctl -c ~/srv/slapos/etc/supervisord.conf shutdown
  supervisorctl -c ~/srv/slapos/inst/etc/supervisord.conf shutdown

Restart slappartX:slapos-standalone-instance-XXX-on-watch from host slapos