[autotest] Record ending time of HostQueueEntry

Add finished_on to afe_host_queue_entries table.
Update the models.
Set the value of finished_on when the hqe is set as completed.

TEST=Run a dummy suite, observe the value is properly set in database.
BUG=chromium:387917
DEPLOY=scheduler, migrate, apache

Change-Id: I462cc0ea508dac6bdc6960b8169199ceb1091086
Reviewed-on: https://chromium-review.googlesource.com/205287
Reviewed-by: Fang Deng <fdeng@chromium.org>
Commit-Queue: Fang Deng <fdeng@chromium.org>
Tested-by: Fang Deng <fdeng@chromium.org>
5 files changed