Specify which file ipxe should load during the netboot.

def pxe_netboot(self, filename):
        """Specify which file ipxe should load during the netboot."""
        new_port = {
            'extra_dhcp_opts': [
                {'opt_name': 'bootfile-name', 'opt_value': 'http://192.0.2.240:8088/' + filename, 'ip_version': 4, },
                {'opt_name': 'tftp-server', 'opt_value': '192.0.2.240', 'ip_version': '4'},
                {'opt_name': 'server-ip-address', 'opt_value': '192.0.2.240', 'ip_version': '4'}
            ]
        }
        self.neutron.update_port(self._provision_port_id, {'port': new_port})