Add InternalIP fallback if no external is found

[ZleFox]

if your cluster has only InternalIP defined(as in rancher rke cluster) nats-boot-config will fallback to it. Another approach is to check for labels first and then falling back to the InternalIP ...
fixes: https://github.com/nats-io/k8s/issues/9

[ekmobile]

@wallyqs @ZleFox any info on how and when the fix will be released? Right now we cannot use NATS on a new AKS Cluster. Thanks.