if we have host2 inside host1 inside host0 then routing from host0 to host2 needs to specify 'via' address because the destination address (of host2) is not directly seen through slaptap of host0 (because it goes only to host1).

this commit introduces a new constraint: users must now put the IPv6 address `network::1` on their ethernet interface inside the VM.

/cc @jm @rafael @alain.takoudjou

/reviewed-on nexedi/slapos.core!102