Map<String, String> clusterConfig() {
        Map<String, String> result = configData.get(DEFAULT_NODE);
        return (result == null) ? Collections.emptyMap() : result;
    }
Get the config value for the give topic and give config key.
If the config value is not found, return null.

@param topicName            The topic name for the config.
@param configKey            The key for the config.
metadata/src/main/java/org/apache/kafka/controller/ConfigurationControlManager.java