Returns the private-config file relative to the project directory.

        :returns: path to private-config file. None if the file doesn't exist

def relative_private_config_file(self):
        """
        Returns the private-config file relative to the project directory.

        :returns: path to private-config file. None if the file doesn't exist
        """

        path = os.path.join(self.working_dir, 'private-config.cfg')
        if os.path.exists(path):
            return 'private-config.cfg'
        else:
            return None