Implement FileWriterPlugin Metadata Fix Issue 408

[Ohisemega]

FileWriterPlugin Metadata Implementation

[Ohisemega]

@ajgdls and @GDYendell, I was wondering if we need an associated method to clean up the configuration metadata parameter container in FileWritePlugin. This is because we have a method FileWriterPlugin::delete_datasets() which cleans the dataset_ defs_ map in the plugin. The logic follows that if the dataset definition is "purged", then the associated metadata parameters in the metadata map container are invalid too. What do you think?

/**
 * Deletes all dataset definitions from the plugin.
 */
void FileWriterPlugin::delete_datasets()
{
  LOG4CXX_INFO(logger_, "Deleting all datasets from FileWriter plugin");
  dataset_defs_.clear();
}