pretty print json in vdsm persistency files

Adding indentation is making the json a lot more human readable

Change-Id: Ib4f3b55e76d4dac523a696c4a55734116761bdc2
Signed-off-by: ibarkan <ibarkan@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/34990
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>