The kubelet uses two different locations to store certificates on
initial bootstrap and then on subsequent rotation:

* bootstrap: certDir/kubelet-client.(crt|key)
* rotation:  certDir/kubelet-client-(DATE|current).pem

Bootstrap also creates an initial node.kubeconfig that points to the
certs. Unfortunately, with short rotation the node.kubeconfig then
becomes out of date because it points to the initial cert/key, not the
rotated cert key.

Alter the bootstrap code to store client certs exactly as if they would
be rotated (using the same cert Store code), and reference the PEM file
containing cert/key from node.kubeconfig, which is supported by kubectl
and other Go tooling. This ensures that the node.kubeconfig continues to
be valid past the first expiration.