GitLab

Get bucket from sdk-bucket.txt unless specifically overriden.
Replace --bucket flag with --bucket-override flag
Remove nargs from --boot-images, which was causing weird flag behavior.

Change-Id: I56ef36f76234f7db5ed57b53401ed90dc528391c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453493
Commit-Queue: Chong Gu <chonggu@google.com>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Auto-Submit: Chong Gu <chonggu@google.com>
Cr-Commit-Position: refs/heads/master@{#814497}