Devserver: Allow xbuddy translate to take in an image_dir kwarg.

Currently xbuddy is tied to gs://chromeos-image-archive. This change
allows us to specify the image directory in Google Storage to use
when requesting a translation.

BUG=chromium:359472
TEST=Able to find the latest build in other buckets.
TEST=xbuddy_unittests.

Change-Id: Iddc2ab2b0675591b53311ebe9e1692d0c0859377
Reviewed-on: https://chromium-review.googlesource.com/200683
Reviewed-by: Simran Basi <sbasi@chromium.org>
Tested-by: Simran Basi <sbasi@chromium.org>
Commit-Queue: Simran Basi <sbasi@chromium.org>
4 files changed