Add HTTP proxy config for client downloads

Client downloads timeout when the device under test is behind a proxy. This
introduces an optional config setting which is then picked up in file_utils.

BUG=chromium:496264
TEST=`test_that ${DUT_IP} platform_Crouton video_PlaybackPerf.vp8`

Change-Id: Ibea4414314c32395f86ab8268449d3a6fb6cc68b
Reviewed-on: https://chromium-review.googlesource.com/274846
Commit-Ready: Edward Baker <edward.baker@intel.com>
Tested-by: Edward Baker <edward.baker@intel.com>
Reviewed-by: Dan Shi <dshi@chromium.org>
2 files changed