public HostSpec hostByMembership(String clusterId, int group, int index) {
            for (HostSpec host : allHosts) {
                if ( ! host.membership().isPresent()) continue;
                ClusterMembership membership = host.membership().get();
                if (membership.cluster().id().value().equals(clusterId) &&
                    groupMatches(membership.cluster().group(), group) &&
                    membership.index() == index)
                    return host;
            }
            return null;
        }
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