include ipv6 address for node external IPs

[AshleyDumaine]

This updates Nodes to also have their ipv6 address set for their external IPs to support dual-stack / ipv6-enabled clusters.

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2. Have you added tests?
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[rahulait]

LGTM.

With vpcless installs:

KUBECONFIG=./kubeadm8.yaml k get node kubeadm8-control-plane-96m6p -o yaml | yq .status.addresses
- address: kubeadm8-control-plane-96m6p
  type: Hostname
- address: 172.233.218.251
  type: ExternalIP
- address: 2600:3c06::f03c:94ff:fe1e:39c7
  type: ExternalIP
- address: 192.168.146.232
  type: InternalIP

With VPC installs:

KUBECONFIG=./kubeadm1.yaml k get node kubeadm1-control-plane-vt8hf -o yaml | yq .status.addresses
- address: kubeadm1-control-plane-vt8hf
  type: Hostname
- address: 10.0.0.2
  type: InternalIP
- address: 192.168.150.247
  type: InternalIP
- address: 172.234.216.122
  type: ExternalIP
- address: 2600:3c06::f03c:94ff:fe1e:cf11
  type: ExternalIP