openvpn: T3214: allow configuring server with v6 only

Starting with v2.5.0 OpenVPN allows configuring a server with
an IPv6 only tunnel.

For this reason there is no need to depend on the existence of an IPv4
subnet anymore.

Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>
(cherry picked from commit 3072e507eb1cdc18cfe5429fd0c03d223d2576fe)