Don't assume bucket name to start with 'master.'.

NOTRY=True
BUG=chromium:617627

Change-Id: I4e634fd2f485ac06c8e06af2de0cd86dbf425e9b
Reviewed-on: https://chromium-review.googlesource.com/355990Reviewed-by: Jamie Madill <jmadill@chromium.org>