cyclades: extend VM info with 'port_forwarding'
Extend servers info API response with 'SNF:port_forwarding' attribute, describing port fowarding rules (DNAT) that are applied to vms. The description of such rules is done via the new CYCLADES_PORT_FORWARDING setting. This setting can be either a dictionary mapping to ports to tuples of (host, port), or a callable object that must return such a tuple. The caller will pass to the callable the following positional arguments, in the following order: * server_id: The ID of the VM in the DB * ip_address: The IPv4 address of the public VM NIC * fqdn: The FQDN of the VM * user: The UUID of the owner of the VM