Add a relay-v6.tmpl template file

For some reasons, relay.tmpl includes exit-v6.i which now requires ipv6_addr to be set else None is put in place.

This makes basic-min fails because it ends up with ORPort None:5001 IPv6Only. Instead, make relay.tmpl to be IPv4 only and relay-v6.tmpl to be v4 and v6.

This makes all network configuration using relay.tmpl to work normally with IPv4 only.