use drain node instead of delete node in rke up

[liuliqiang]

RKE version:

v0.3.0-rc3

These days,i review the codes of rke to find out how rke remove a nodes, and i get the step for rke to remove a node is following:

https://github.com/rancher/rke/blob/v0.3.0-rc3/hosts/hosts.go#L148

func DeleteNode(ctx context.Context, toDeleteHost *Host, kubeClient *kubernetes.Clientset, hasAnotherRole bool, cloudProvider string) error {
	if hasAnotherRole {
		log.Infof(ctx, "[hosts] host [%s] has another role, skipping delete from kubernetes cluster", toDeleteHost.Address)
		return nil
	}
	log.Infof(ctx, "[hosts] Cordoning host [%s]", toDeleteHost.Address)
	if _, err := k8s.GetNode(kubeClient, toDeleteHost.HostnameOverride); err != nil {
		if apierrors.IsNotFound(err) {
			log.Warnf(ctx, "[hosts] Can't find node by name [%s]", toDeleteHost.Address)
			return nil
		}
		return err

	}
	if err := k8s.CordonUncordon(kubeClient, toDeleteHost.HostnameOverride, true); err != nil {
		return err
	}
	log.Infof(ctx, "[hosts] Deleting host [%s] from the cluster", toDeleteHost.Address)
	if err := k8s.DeleteNode(kubeClient, toDeleteHost.HostnameOverride, cloudProvider); err != nil {
		return err
	}
	log.Infof(ctx, "[hosts] Successfully deleted host [%s] from the cluster", toDeleteHost.Address)
	return nil
}

maybe it better to remove nodes by using kubectl drain way?

[superseb]

This will be improved in #1800