[autotest] Deploy servers in parallel

BUG=chromium:496435
TEST=local run
site_utils/deploy_production.py --dryrun -v
inject following in update_server (before return) to simulate a push
    import time, random
    r=random.random()*10
    if r<3:
        return server, False, 'error'
    else:
        time.sleep(r)

Change-Id: Id83a37b973aab7627b65f23aade0118823ff21fa
Reviewed-on: https://chromium-review.googlesource.com/276660
Trybot-Ready: Dan Shi <dshi@chromium.org>
Tested-by: Dan Shi <dshi@chromium.org>
Reviewed-by: Dan Shi <dshi@chromium.org>
Commit-Queue: Dan Shi <dshi@chromium.org>
2 files changed