Revert "Revert "GS Cache: using vpython""

This reverts commit 30c6bbd0a05990b744de7ba9f501973172d612a7.

The original CL got reverted due to a bug in chromite. Now it has been
fixed in crosreview/1050876. Thus this CL can re-merged.
The original review for the CL is crosreview/1038646.

BUG=chromium:824580
TEST=None

Change-Id: I6112af5d9d71205dcadebe4223a60c3a2526a779
Reviewed-on: https://chromium-review.googlesource.com/1056158
Commit-Ready: Congbin Guo <guocb@chromium.org>
Tested-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
7 files changed