Allow multi control plane clusters by knowing which node is the api server

[sharifelgamal]

Right now we assume if a node is a control plane it's automatically is the api server, and that's not necessarily the case so we need to make sure the api server node is explicitly specified.

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[lingsamuel]

/assign
I almost done it. But I think Kubeadm is not very friendly to this work at present. I will submit some PRs to k/k later to make the implementation of this feature more convenient.