public ConfigurationChanges remove(String propertyName) {
      PropertyChange propertyChange = new PropertyChange(ChangeType.REMOVE, propertyName, null);
      m_changes.add(propertyChange);
      return this;
    }
Marks a configuration property for removal.

@param propertyName
         the name of the property to remove.
@return an instance of this class with the property marked for removal.
ambari-server-spi/src/main/java/org/apache/ambari/spi/upgrade/UpgradeActionOperations.java