Make Cyclades support IPv6 only networks. Such networks will have
the 'subnet' attribute set to None, in the same way that IPv4 only
networks have the 'subnet6' attribute set to None.

Because currently Ganeti does not support IPv6 only networks, we create the
corresponding Ganeti network with a dummy IPv4 subnet(10.0.0.0/24) that will be
never used, since Cyclades will connect instances to IPv6 networks with
'address' attribute set to None.