Ignore postZero flag when deleting an image on a fileSD.

Ignoring parameters is very bad but we will continue to do that
because fixing the engine is very hard.

Bug-url: https://bugzilla.redhat.com/show_bug.cgi?id=911209

Change-Id: Ief50d6174fd20e16f2fdf48a6cd512d20b9f407d
Signed-off-by: Eduardo <ewarszaw@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/12404
Reviewed-by: Yeela Kaplan <ykaplan@redhat.com>
Reviewed-by: Ayal Baron <abaron@redhat.com>
Tested-by: Daniel Paikov <paikov@gmail.com>