@Override
    public Optional<StorageNode> storageNodeInGroup() {
        return storageNodeInGroup(serviceInstance-> true);
    }
A description of the hosts outside the group that are allowed to be down,
and a description of the services outside the group and outside of the allowed services
that are down.
orchestrator/src/main/java/com/yahoo/vespa/orchestrator/model/ClusterApiImpl.java