Allow specifying node labels in configuration.

Specifically for cases where using `installFlags: ["--labels=whatever"]` isn't acceptable, e.g.

Kubelets can't set the `node-role.kubernetes.io/master=""` label on themselves for security reasons,
it has to be done via an API client (e.g. kubectl).
See https://github.com/kubernetes/kubernetes/issues/84912#issuecomment-551362981

This could be added as the following:
```yaml
spec:
hosts:
- role: controller+worker
labels:
- "node-role.kubernetes.io/master="
```

or

```yaml
spec:
hosts:
- role: controller+worker
labels:
- key: node-role.kubernetes.io/master
value: ""
```

The issue names no file or test; begin by tracing how k0sctl parses host configuration and applies node changes through Kubernetes. Compare the two proposed label forms, then verify that configured labels are applied to the target node and add coverage for the chosen format.

Written by the indexing model from the issue text.