Fix debug parameter

-d was removed from cdist in favor of mulitple -v and -l parameters, but
-d was not removed from preos.

Resolve #849.

cdist/preos/debootstrap/files/code

@@ -22,7 +22,7 @@ set -e
 if [ "${debug}" ]
 then
     set -x
-    cdist_params="${cdist_params} -d"
+    cdist_params="${cdist_params} -l 3"
 fi
 
 bootstrap_dir="${target_dir}"