openvpn: T3214: warn when setting nopool and server-ipv6 is being used

Currently OpenVPN does not allow having an IPv6 subnet if 'nopool'
was specified on the --server directive. For this eason warn if this
specific configuration is being hit.

This is probably something that should be fixed upstream, but for now
we can't allow this combination of parameters.

Signed-off-by: Antonio Quartulli <antonio@mandelbit.com>