buildbucket: non-transactional bucket access
buildbucket: non-transactional bucket access

Mark config.get_buckets and config.get_bucket functions with
@ndb.non_transactional so Bucket entities are not accessed in a
transactional of a different entity group.

R=vadimsh@chromium.org
BUG=

Review URL: https://codereview.chromium.org/1152753004

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra