obs/slapos: Remove only configuration and logs on purge

Policy does not seems to enforce data removal on purge:

https://www.debian.org/doc/debian-policy/ch-files.html

So let's keep data like supplied software and deployed instances.

Also add a fix if ucf command not present.

See merge request !208

Co-authored-by: Thomas Gambier <thomas.gambier@nexedi.com>