Respect `WHEREABOUTS_KUBECONFIG_FILE_HOST` env var

[akhilles]

From what I can tell, it's currently defined in ./script/install-cni.sh but ignored. So, it's not possible to force the CNI to use an existing kubeconfig (e.g. /etc/kubernetes/kubelet.conf).

Also, if there's already a kubeconfig file at WHEREABOUTS_KUBECONFIG_FILE_HOST, I think it's reasonable to just use it and not overwrite it.