Comment 2 for bug 1767303

If https://review.openstack.org/#/c/539617/ isn't enough, we could also think about adding a HostMapping.uuid field which mirrors the ComputeNode.uuid field and then we could get the list of host mappings by uuids and from that list get the list of cells from which to pull the compute nodes, but it would be good to know if https://review.openstack.org/#/c/539617/ made a big enough difference that it's good enough for now.