private void waitForAllFutures(List<Entry<String, Future<?>>> futureEntries)
            throws Exception {
        for (Entry<String, Future<?>> entry : futureEntries) {
            log.debug("waiting for {} to shut down.", entry.getKey());
            entry.getValue().get();
            log.debug("{} successfully shut down.", entry.getKey());
        }
    }
Wait for a controller to mark all the brokers as ready (registered and unfenced).
And also wait for the metadata cache up-to-date in each broker server.
core/src/test/java/kafka/testkit/KafkaClusterTestKit.java