replace `kubeadm alpha kubeconfig user` with `kubectl config` commands

[chuckha]

Describe the solution you'd like
Instead of depending on an alpha kubeadm feature that is at best staying unchanged in the alpha phase forever and at worst getting deleted (likely), we should get ahead of this curve and replace that feature with kubectl config commands.

I think it will be something like

1. create kubeconfig with kubeadm
2. set it as the active kubeconfig
3. modify the config with the correct server & user

Anything else you would like to add: