[infra] Make GS buckets for images and hash file URL configurable

This is necessary for lottie-ci so we can upload to different
buckets and fetch different hash files.

Bug: skia:
Change-Id: I98b4d551df34473e2b5fc04284b0de04fbc097f3
Reviewed-on: https://skia-review.googlesource.com/144840
Commit-Queue: Stephan Altmueller <stephana@google.com>
Reviewed-by: Eric Boren <borenet@google.com>
53 files changed