Show an alert on knictl about destroy bootstrap vm

Currently, worker nodes cannot read correctly the ignition endpoint, when the bootstrap VM is active (they give an internal server error, see https://github.com/openshift/okd/issues/174 ).
So we need to alert that the bootstrap vm needs to be destroyed right after "deploy_workers" command is started.
I'd like to reflect that in the message that we display in knictl, and also on the wiki. For displaying in knictl, it can be added on: https://github.com/akraino-edge-stack/kni-installer/blob/master/pkg/manifests/manifests.go#L223