[autotest] Get atest to list the shard a host is on.

TEST=Ran atest host list.
BUG=chromium:438858

Change-Id: I0ec46f1d3f8068c0f2144a8a0e5a769cf81a0c96
Reviewed-on: https://chromium-review.googlesource.com/235545
Reviewed-by: Prashanth B <beeps@chromium.org>
Tested-by: Prashanth B <beeps@chromium.org>
Commit-Queue: Prashanth B <beeps@chromium.org>
Trybot-Ready: Prashanth B <beeps@chromium.org>
3 files changed