Re-apply 3055916b5ddcea62885a637c3a26a4e7d75b6107 correcting the copy/paste error. Refs #2563.

We should now provide the configuration filename to load configurations from at the parser's class level.
The calls to `ConfigDirMixIn.get_config_file_path()` still accept a filename, though, if that filename is `None`(the default), it grabs the filename from the class level attribute `_config_filename_`.