In case stacked etcd is used, the code that does expiration checks
does not validate if the etcd CA is "external" (missing key)
and if the etcd CA signed certificates are valid.

Add a new function UsingExternalEtcdCA() similar to existing functions
for the cluster CA and front-proxy CA, that performs the checks for
missing etcd CA key and certificate validity.

This function only runs for stacked etcd, since if etcd is external
kubeadm does not track any certs signed by that etcd CA.

This fixes a bug where the etcd CA will be reported as local even
if the etcd/ca.key is missing during "certs check-expiration".

Apply a small fix to ensure the kubeconfig files
that kubeadm manages have a CA when printed in the table
of the "check expiration" command. "CAName" is the field used for that.

In practice kubeconfig files can contain multiple credentials
from different CAs, but this is not supported by kubeadm and there
is a single cluster CA that signs the single client cert/key
in kubeadm managed kubeconfigs.