Returns the content of the current startup-config file.

def startup_config_content(self):
        """
        Returns the content of the current startup-config file.
        """

        config_file = self.startup_config_file
        if config_file is None:
            return None

        try:
            with open(config_file, "rb") as f:
                return f.read().decode("utf-8", errors="replace")
        except OSError as e:
            raise IOUError("Can't read startup-config file '{}': {}".format(config_file, e))