Support a --dry-run mode for all kubeadm functions

[luxas]

Generally, all POST/PUT/PATCH/DELETEs, etc must be discarded in dry-run mode

On kubeadm init, all GET/LISTs will be faked; it will return values that it knows are valid to get the kubeadm init flow working well in dry-run mode.
On kubeadm token, join, upgrade where we can access an API server; it will proxy the GET/LIST to that API Server to get real values for kubeadm to react on.

Regarding the filesystem; a temporary directory will be used for writing files. Then the content will be printed and the temp directory removed

[luxas]

This is not yet fully fixed, sending a PR now...