Kubespan shouldn't report 'external' IPs as node addresses

[smira]

When using "external" IPs (cloud-assigned public IPs which are not assigned to the node networking stack), KubeSpan shouldn't report those as node addresses reachable over KubeSpan.

The problem with external IPs is that if the packet for external IP reaches the proper node, it will be dropped, as the node doesn't have that address assigned to the node. Those external IPs are only reachable when going over public Internet.