During "join" of new control plane machines, kubeadm would
download shared certificates and keys from the cluster stored
in a Secret. Based on the contents of an entry in the Secret,
it would use helper functions from client-go to either write
it as public key, cert (mode 644) or as a private key (mode 600).
The existing logic is always writing both keys and certs with mode 600.
Allow detecting public readable data properly and writing some files
with mode 644.
First check the data with ParsePrivateKeyPEM(); if this passes
there must be at least one private key and the file should be written
with mode 600 as private. If that fails, validate if the data contains
public keys with ParsePublicKeysPEM() and write the file as public
(mode 644).
As a result of this new logic, and given the current set of managed
kubeadm files, .key files will end up with 600, while .crt and .pub
files will end up with 644.
* Upload certificates: generate certificates and check that the kubeadm-certs
secret is populated with the expected contents.
* Download certificates: given we have a kubeadm-certs secret, ensure that
keys and certificates are copied on the expected target, and that depending
on the secret nature they have the expected permissions.
This implements the certificate download for the join process. If certificates
have been uploaded during init (or explicitly on any master node) and the secret
is still present in the cluster, the join process will try to download the secret
data, decrypting it with the provided key in the new `--certificate-key` flag.
