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 !208Co-authored-by: Thomas Gambier <thomas.gambier@nexedi.com>

obs/slapos/debian/postrm

@@ -5,10 +5,10 @@ set -e
 
 case "$1" in
     purge)
-        ucf --purge /etc/opt/slapos/slapos.cfg
+        command -v ucf >/dev/null && ucf --purge /etc/opt/slapos/slapos.cfg
 
-        rm -rf /var/opt/slapos/ /srv/slapgrid/ /opt/slapgrid/ \
-            /etc/opt/slapos/ /opt/slapos/
+        rm -rf /var/opt/slapos/ /etc/opt/slapos/ /opt/slapos/
+        echo "Note: purging slapos-node removed the configuration files and the logs. If you want to also remove the data, run 'rm -rf /opt/slapgrid /srv/slapgrid'."
      ;;
 
      remove)