Fix, config_file_name was ignored

There are two small fixes in this PR.
1. The default hard coded value was removed from *config_file_name* since that prevented *_config_file_name_default* from ever being called (which makes it pointless).  Also the *_config_file_name_default* method pro grammatically generated the same value.
2. The hard coded value for *_config_files_default* was replaced with a reference to *config_file_name*.  If this isn't done, *config_file_name* never gets referenced and is thus useless.

I was forced to deal with this when I found *self.config_file_name = Unicode(u'something.py')* was no longer working for classes inheriting from *BaseIPythonApplication*