autotest: Add logic to log netperf keyvals

We'll have to do something slightly different when we get performance
dashboard integration, but this does a nice job of generating reports in
the meantime.

TEST=netperf tests log nice results.
BUG=chromium:268643
Change-Id: Ica4cc75e4986b3fb14894e1907e7b6cab5f9722a
Reviewed-on: https://gerrit.chromium.org/gerrit/64963
Tested-by: Christopher Wiley <wiley@chromium.org>
Reviewed-by: mukesh agrawal <quiche@chromium.org>
Commit-Queue: Christopher Wiley <wiley@chromium.org>
4 files changed