Automatic merge from submit-queue
[RFC] Prepare for deprecating NodeLegacyHostIP
Ref https://github.com/kubernetes/kubernetes/issues/9267#issuecomment-257994766
*What this PR does*
- Add comments saying "LegacyHostIP" will be deprecated in 1.7;
- Add v1.NodeLegacyHostIP to be consistent with the internal API (useful for client-go migration #35159)
- Let cloudproviders who used to only set LegacyHostIP set the IP as both InternalIP and ExternalIP
- Master used to ssh tunnel to node's ExternalIP or LegacyHostIP to do [healthz check](https://github.com/kubernetes/kubernetes/blame/master/pkg/master/master.go#L328-L332). OTOH, if on-prem, kubelet only [sets](https://github.com/kubernetes/kubernetes/blob/master/pkg/kubelet/kubelet_node_status.go#L430-L431) LegacyHostIP or InternalIP. In order to deprecate LegacyHostIP in 1.7, I let healthz check to use InternalIP if ExternalIP is not available. (The healthz check is the only consumer of LegacyHostIP in k8s.)
@liggitt @justinsb @bgrant0607
```release-note
LegacyHostIP will be deprecated in 1.7.
```
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