Enhance factory bundle script to download Bigstore images using Gsutil

1. investigated alternatives (which would not work here)
a.) OAuth 2.0 Python API for Google Storage for Developers (BigStore)
b.) corporate SSO
2. user is required to download/install/configure "gsutil" in order to
   take advantage of this enhancement
a.) this is one-time set-up and gsutil will handle access token
    acquisition/refresh
3. refactored existing code to be more concise/readable
4. remove unusable functional test for cros_bundle.py
5. update unit tests to provide coverage for new code

BUG=chrome-os-partner:6298
TEST=manual

Change-Id: I5ab2fb0c3906c7920a7a2b26b091c6590315e31f
Reviewed-on: http://gerrit.chromium.org/gerrit/9960
Reviewed-by: Ang Pan <oscarpan@chromium.org>
Tested-by: Tan Gao <tgao@chromium.org>
Commit-Ready: Tan Gao <tgao@chromium.org>
9 files changed