iperf3: Avoid using --logfile arg

Some older iperf3 versions don't support the --logfile arg. Let's
instead parse the json from stdout. Old --logfile behavior is left in
code but disabled since it's a nicer (less hacky) way to get results,
because parsing from stdout means we need to avoid reading some content.

Change-Id: Ia1b7fa4dae074089878963aeddf0006aea68e31f
1 file changed