code: public T setConf(String key, String value) {
      checkNotNull(key, "key");
      checkNotNull(value, "value");
      checkArgument(key.startsWith("spark."), "'key' must start with 'spark.'");
      builder.conf.put(key, value);
      return self();
    }
docs: Set a single configuration value for the application.
  
  @param key Configuration key.
  @param value The value to use.
  @return This launcher.