caddy-frontend-is-running-actual-software-release promise is not needed
anymore, as hash-files are used.

There is no need anymore to have two processes for normal and nginx slaves,
as nginx ones are served by caddy anyway.

Also inform the requester that type:eventsource is not implemented.

/reviewed-on nexedi/slapos!518

As check-free-disk-space promise is implemented in plugin directory, add it
to list of assertions and drop it from promise list assertions.

Also already existing monitoring plugins are asserted.

/reviewed-on nexedi/slapos!368