Merge lp://staging/~rodsmith/hwcert-tools/multi-iperf3 into lp://staging/~leftyfb/hwcert-tools/cert-precheck

Mike,

I've looked over your pre-certification script, and have these changes for you. The issue is that your script didn't parse multiple iperf targets in /etc/xdg/canonical-certification.conf, or check for iperf3 servers. My patch does that. A few notes:

* It mostly-replicates the iperf2 code to test iperf3. I'd rather do a loop, but that started to get hairy in various ways, so I left it with mostly-replicated code instead.
* I use an iperf3 call to check for the presence of the server rather than nc because I discovered that the iperf3 server terminates after several unsuccessful connection attempts by nc, so the test could end up causing problems if nc were used. This has the drawback that the script will pause for a bit if an IP address is invalid, but otherwise it seems to work fine.
* My change is a little verbose in what it reports, but trimming it more might make it less useful.