- Extend the Network model with fields for holding the IPv6 subnet and
  gateway.
- Update network API methods.
- Modify ganeti hooks to return IPv6 address for all network interfaces.
  IPv6 is now derived from the IPv6 subnet of the network in Ganeti,
  instead of the old setting (PUBLIC_IPV6_PREFIX)
- Remove obsolete setting (`PUBLIC_IPV6_PREFIX`)
- Update management commands for IPv6