devserver: Add quick provision mode for auto updates.

Quick provision mode simplifies provision process to just retrieve
and execute a script on the DUT, where the script is responsible for
retrieving and directly writing partitions to disk.

BUG=chromium:779870
TEST=curl "http://${ds}/stage?archive_url=gs://chromeos-image-archive/${build}&artifacts=quick_provision,stateful"; curl "http://${ds}/cros_au?full_update=False&force_update=True&build_name=${build}&host_name=${dut}&async=False&clobber_stateful=True&quick_provision=True"

Change-Id: I1f907cd16f7da6be8b01dd0c08e85dfbfbe3f82a
Reviewed-on: https://chromium-review.googlesource.com/751792
Commit-Ready: David Riley <davidriley@chromium.org>
Tested-by: David Riley <davidriley@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
3 files changed