Add taint flag to cluster add-node command

[mouellet]

On Rancher's cluster registration page, the one that shows you de node registration command, you have the options (Step 2) to add both node labels and taints. However, the cli cluster add-node command, which also prints the node registration command, only has a flag for node labels.

This Pull Request adds a flag for node taints to the cluster add-node command, printing a node registration command accordingly.

rancher cluster add-node --worker --taint key=value:NoSchedule
sudo docker run -d --privileged --restart=unless-stopped --net=host -v /etc/kubernetes:/etc/kubernetes -v /var/run:/var/run  rancher/rancher-agent:v2.6.5 --server https://redacted --token redacted --ca-checksum redacted --worker --taint key=value:NoSchedule

[mouellet]

In light of rancher/rancher#36131, might need to use --taints as flag instead...