API.updateTimestamp: use touchFile

Opening a file for writing with the file() constructor is frowned upon.
Particularly when we have a library function that does the intended
action properly.

Change-Id: I4d48fc0244ec8059d52771f9d0ccc98735ae9aa2
Signed-off-by: Dan Kenigsberg <danken@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/30232
Reviewed-by: Antoni Segura Puimedon <asegurap@redhat.com>
Reviewed-by: Piotr Kliczewski <piotr.kliczewski@gmail.com>
Reviewed-by: Dima Kuznetsov <dkuznets@redhat.com>