GS Cache: using vpython

In this change, I move the implementation of gs_archive_server from venv
directory to a standalone directory and use vpython to run it. In this
way, the server runs in a separate virtual environment.

BUG=chromium:824580
TEST=Call `bin/gs_archive_server` and ensured the service got started.

Change-Id: I64f5ee73ea7723db31a5814d33db0b685fd7bd84
Reviewed-on: https://chromium-review.googlesource.com/1038646
Commit-Ready: Congbin Guo <guocb@chromium.org>
Tested-by: Congbin Guo <guocb@chromium.org>
Reviewed-by: Congbin Guo <guocb@chromium.org>
7 files changed