dufs: use jsonschema and publish upload user and password

Follow up to https://lab.nexedi.com/nexedi/slapos/-/merge_requests/2073 , without the switch-softwaretype and backup features which are not needed anymore /cc @jerome @tomo

I renamed instance.cfg.in to instance.cfg.jinja2 since it is a jinja2 template and not a slapos.recipe.template, I think it is less confusing