The kubeconfig phase of "kubeadm init" detects external CA mode
and skips the generation of kubeconfig files. The kubeconfig
handling during control-plane join executes
CreateJoinControlPlaneKubeConfigFiles() which requires the presence
of ca.key when preparing the spec of a kubeconfig file and prevents
usage of external CA mode.
Modify CreateJoinControlPlaneKubeConfigFiles() to skip generating
the kubeconfig files if external CA mode is detected.
7c783fa374