Create new flag to specify apiserver address for kube-proxy

[discordianfish]

Right now kubeadm uses the host IP it's running on for the apiserver address in the kube-proxy configmap.
For a multi-master setup where the apiserver are behind a load balancer, we need a way to configure kube-proxy to talk to the load balancer instead.
Therefor I propose adding a flag to kubeadm to set the apiserver address in the kube-proxy config map.

This is probably required for #261
Related:

• validate AdvertiseAddress in kubeadm init and other case kubernetes#56956
• Workarounds for the time before kubeadm HA becomes available #546

[fejta-bot]

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[discordianfish]

/remove-lifecycle stale
/lifecycle freeze

[timothysc]

/assign @timothysc @fabriziopandini

[fabriziopandini]

@discordianfish considering the availability of the new API.ControlPlaneEndpoint config option (that could be used also for defining the address of the LB in an HA scenario), is the new flag still needed or not?

[discordianfish]

Oh right, this can be closed now too.