Move buildbucket_build_status constants to lib/constants.py

To track slave builds using buildbucket, we will collect and
process build statuses in different places. lib/constants.py is a better
place to hold the buidlbucket_build_status (like the build statuses from
CIDB).

BUG=chromium:655295
TEST=unit_tests

Change-Id: I3dd72e398b8276ccb3c71340ce5eccb4048c1034
Reviewed-on: https://chromium-review.googlesource.com/399701
Commit-Ready: Ningning Xia <nxia@chromium.org>
Tested-by: Ningning Xia <nxia@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
5 files changed