`public-ip-overwrite` using node-labels

[skirsten]

Currently (as far as I know) the only way to overwrite the public-ip in kubernetes mode is via node annotations. For k3s this means there is no way (I know of) to set the annotation before it will be consumed by the integrated flannel. In this case k3s would have to be restarted to apply the change which is a deal-breaker for me.

This PR simply allows to configure the exact same overwrite via node-labels. If both are present, the annotation is preferred.

I have not tested this locally but I added a functional test.