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kubeadm: set kube-apiserver advertise address using downward API
**What this PR does / why we need it**:
Sets kube-apiserver --advertise-address using downward API for self-hosted Kubernetes with kubeadm.
**Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
Fixes # https://github.com/kubernetes/kubeadm/issues/400
**Special notes for your reviewer**:
@luxas mentioned something about losing the ability to customize advertise address via kubeadm, didn't really take that into consideration yet but I can definitely make the necessary changes if needed.
**Release note**:
```release-note
kubeadm: set kube-apiserver advertise address using downward API
```
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