This is based on a mailing list message from a whyle ago[1] and
we've been making significant progress toward it. There are still
some steps remaining, so include them here, along with the big
picture, as a spec.
[1] http://lists.zuul-ci.org/pipermail/zuul-discuss/2018-July/000511.html
Change-Id: If144269d30a712de602d3187e3ab791a0169575f