Allow specifying the configuration file name at the parser's class level. 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_`.
Merge pull request #3772 from s0undt3ch/issues/2563

Allow specifying the configuration file name at the parser's class level.