public ConfigurationChanges set(String propertyName, String propertyValue) {
      m_onlyRemovals = false;

      PropertyChange propertyChange = new PropertyChange(ChangeType.SET, propertyName,
          propertyValue);

      m_changes.add(propertyChange);
      return this;
    }
Sets either a new or an update to an existing configuration property.

@param propertyName
         the name of the property.
@param propertyValue
         the value for the property.
@return an instance of this class with the value set.
ambari-server-spi/src/main/java/org/apache/ambari/spi/upgrade/UpgradeActionOperations.java