code: public void updateConf(PropertyKey key, @Nullable String value) {
      if (value == null) {
        mProperties.remove(key);
      } else {
        mProperties.put(key, value);
      }
    }
docs: Updates the master's configuration. This update will take effect the next time the master
  is started.
  
  @param key the conf key to update
  @param value the value to set, or null to unset the key