cbuildbot_config: add buildbot_waterfall_name field

This field will hint to buildbot what builder name to use on the
waterfall.

BUG=chromium:456200
TEST=New unit test

Change-Id: I4b978edec42eef24c63a244bddd8f1a99ef14f3e
Reviewed-on: https://chromium-review.googlesource.com/247380
Trybot-Ready: Aviv Keshet <akeshet@chromium.org>
Tested-by: Aviv Keshet <akeshet@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Aviv Keshet <akeshet@chromium.org>
3 files changed