Set the kubelet `--hostname-override` flag conditionally at kubeadm init time

[luxas]

When kubernetes/kubernetes#63887 merges, we should set the kubelet's --hostname-override flag to what's passed to kubeadm via the --node-name flag conditionally if cfg.NodeName != nodeutil.GetHostname("") (if the flag differs from the default) at kubeadm init time, in cmd/kubeadm/app/phases/kubelet/flags.go.

This will automagically fix #584
xref related issue: #822
/assign @liztio
as you're assigned to #584. LMK if you don't have time to do this.

[detiber]

There is another tricky little aspect to this. Since we currently deploy kube-proxy as a daemonset with a single config map, we also need to have a way to pass this value along to the kube-proxy config as well, otherwise Services of type NodePort or LoadBalancer configured with externalTrafficPolicy: local will fail.

[luxas]

@detiber do you have more information on this somewhere? Would this mean every kube-proxy daemon needs to be configured differently?

[detiber]

@luxas I created a separate issue to track the kube-proxy related config: #857