public static HostName getControllerHostName(ApplicationInstance application, ClusterId contentClusterId) {
        //  It happens that the master Cluster Controller is the one with the lowest index, if up.
        return getClusterControllerInstancesInOrder(application, contentClusterId).stream()
                .findFirst()
                .orElseThrow(() ->
                        new IllegalArgumentException("No cluster controllers found in application " + application));
    }
@return  Host name for a Cluster Controller that is likely to be the master.
@throws  java.lang.IllegalArgumentException if there are no cluster controller instances.
orchestrator/src/main/java/com/yahoo/vespa/orchestrator/model/VespaModelUtil.java