Use the helper VM's tmp to decode injected files

Till now the injected files were decoded in the tmp directory of the
instance. We did this because we used to chroot to install the files
with correct ownership. Since chrooting in abandoned, there is no need
to do this any more.

snf-image-helper/tasks/60EnforcePersonality.in

 #! /bin/bash
 
-# Copyright (C) 2011 GRNET S.A. 
+# Copyright (C) 2011-2015 GRNET S.A. 
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -52,7 +52,7 @@ if [ "$SNF_IMAGE_PROPERTY_OSFAMILY" = "windows" ]; then
     exit 0
 else
 
-    tmpdir="$(env TMPDIR="$SNF_IMAGE_TARGET/tmp" mktemp -d)"
+    tmpdir=$(mktemp -d)
     add_cleanup rm -rf "$tmpdir"
     echo "$SNF_IMAGE_PERSONALITY" |
         @scriptsdir@/inject-files.py -d "$tmpdir"