Fixes #792

Allow setting the generated kubeconfig username using --user in k0sctl kubeconfig command. Alternatively the username can be set in k0sctl.yaml metadata.user. The cluster name can also be set using --cluster flag.

When using --kubeconfig-out in apply, the user can be set via --kubeconfig-user and the cluster name via --kubeconfig-cluster.

Example:

$ k0sctl kubeconfig --user example --cluster example-cluster > example-kubeconfig
$ kubeconfig --kubeconfig example-kubeconfig --user example --cluster example-cluster get nodes