[playbook] Use mktemp for temporary location

Instead of chosing arbitrary directory and removing, often not
accessible directories, use standard mktemp for temporary location.

/reviewed-on nexedi/slapos.package!28

playbook/util/diff-playbook

 HERE=`pwd`
-rm -rf /tmp/tmpplaybook_*
-DFILE="/tmp/tmpplaybook_unstable$(basename $0).$$/"
+DFILE=`mktemp -d`
 TFILE="archive.tar.gz"
-mkdir -p $DFILE
 cd $DFILE
 slapcache-download --destination=$TFILE
 tar -xzf $TFILE