fileSD: unify the metadata permissions definition

This refactoring is unifying the metadata permissions definition in one
constant.

Change-Id: I7e5664b88e92237c9b1f903b5142c3f2bd6ba953
Signed-off-by: Federico Simoncelli <fsimonce@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/26413
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-by: Allon Mureinik <amureini@redhat.com>
Reviewed-by: Francesco Romani <fromani@redhat.com>