Output text to `add-node` command can be improved

[pmatulis]

I installed 1.31 today and the output to the add-node command is confusing:

microk8s add-node

I'm told:

From the node you wish to join to this cluster, run the following:
microk8s join 10.54.53.229:25000/0c7f86c53691b2d01abba161b69ee7c1/ca5f25c53b42

Use the '--worker' flag to join a node as a worker not running the control plane, eg:
microk8s join 10.54.53.229:25000/0c7f86c53691b2d01abba161b69ee7c1/ca5f25c53b42 --worker

If the node you are adding is not reachable through the default interface you can use one of the following:
microk8s join 10.54.53.229:25000/0c7f86c53691b2d01abba161b69ee7c1/ca5f25c53b42

A couple of things:

1. The words "one of the following" intimates strongly that the user should expect more than one command. There is only one given.
2. The command I did get (3rd command) was identical to the 1st command.