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

When running tasks, shell is invoked like:

   $THEIA_SHELL -c "command with arguments that\ can\ contain\ spaces"

by logging the process output

After trying to generate a random image ourselve with image magic with:

    ${imagemagick:location}/bin/convert -size 6x6 xc: +noise Random \( -clone 0 -flip \) -append \( -clone 0 -flop \) +append -scale 128x128 $${:location}

I gave up and just download an image from gravatar, but ignoring errors
if any to comply with the "instanciation should work offline" rule.

like webrunner is doing

So that we can access over IPv4 with a valid certificate

This required to generate a certificate, otherwise accessing from
frontend cause 404 site not served on this interface

Remove method `generateHashFromFiles` in all file `test.py`, import it from `slapos.recipe.librecipe`.

/reviewed-on nexedi/slapos!617

Use xz instead of gzip.
Also put the cron script in etc/service instead of etc/run (to have the
"cron-on-watch" process)

/reviewed-on nexedi/slapos!350