devserver: Add support for control_files.tar and autotest_packages.tar

We are splitting up the autotest.tar tarball into 2 separate tarballs.
This change updates the devserver code so that it is able to properly
stage these artifacts.

BUG=chromium:421122
TEST=local devserver and local moblab build that tested a trybot image
with no autotest.tar artifact.

Change-Id: I546add63b74906ce2969e45f8209e074bed02b5c
Reviewed-on: https://chromium-review.googlesource.com/226822
Reviewed-by: Prashanth B <beeps@chromium.org>
Reviewed-by: Yu-Ju Hong <yjhong@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
2 files changed