private boolean groupMatches(Optional<ClusterSpec.Group> clusterGroup, int group) {
            if ( ! clusterGroup.isPresent()) return group==0;
            return clusterGroup.get().index() == group;
        }
Returns a host by cluster name and index, or null if there is no host with the given values in this
node-repository/src/test/java/com/yahoo/vespa/hosted/provision/provisioning/ProvisioningTest.java