smoketest: openvpn: T5270:

This fixes the smoketest after the change in commit e7d7bd20b ("openvpn: T5270:
do not require classic DH params in any more Generate 'dh none' instead and let
OpenVPN use ECDH")

... as there is no exception raised

05:47:26 DEBUG - ======================================================================
05:47:26 DEBUG - FAIL: test_openvpn_server_verify (__main__.TestInterfacesOpenVPN.test_openvpn_server_verify)
05:47:26 DEBUG - ----------------------------------------------------------------------
05:47:26 DEBUG - Traceback (most recent call last):
05:47:26 DEBUG - File "/usr/libexec/vyos/tests/smoke/cli/test_interfaces_openvpn.py", line 342, in test_openvpn_server_verify
05:47:26 DEBUG - with self.assertRaises(ConfigSessionError):
05:47:26 DEBUG - AssertionError: ConfigSessionError not raised