Despite we were checking for the kubelet kubeconfig file to be present, the
kubelet first writes this file and then the certificates the kubeconfig file
refers to. This represents a race condition in kubeadm in which when we confirm
that the kubelet's kubeconfig file is present we continue creating a clientset
out of it. However, the clientset creation will ensure that the certificates the
kubeconfig file refers to exist on the filesystem.

To fix this problem, not only wait for the kubelet's kubeconfig file to be
present, but also ensure that we can create a clientset ouf of it on our polling
process, while we wait for the kubelet to have performed the TLS bootstrap.