Initial ipv6 / iptables work

More for discussion than for actual merging (for now).

I think IPv6 could solve the address allocation problem that seems to be holding k8s back on EC2.  EC2 "tolerates" IPv6 for internal networking by using protocol 41 encapsulation.

I'm currently working on getting cluster/kube-up.sh to work, but in the meantime feedback on this idea would be helpful!
Ah - gotcha!  Yes, random is just a cheap-and-cheerful way of implementing (probably) don't-reuse

The only other thing is that random also avoids trivially disclosing how many other instances are running, which is important in some shared environments.