Join nodes from within the control plain server

**What do you want?**

Be able to join a node from the control plain server.

**Why do you want this?**

**TL;DR I want to be able to join nodes from within the control plain server.**

When attempting to join a node to the cluster I was running k3sup from the control server and provided the necessary flags I was unable to do so because k3sups assumes ssh-ing into the control plain server to ssh into the nodes and join them. I use a different ssh key for the nodes to the control plain and this causes an issue with trying to use my node ssh keys for sshing into the control plain while already sshed into the control plain.

**Recommended solution**

I would like to add an additional flag such as `--local` to indicate the join command doesn't need to ssh into control plain server and just ssh into node and join it.

Alternatively add a flag `--server-ssh-key` that will work with `--server-ip`, `--ssh-key` to allow using different ssh key for the control plain and nodes.

**Additional context**

Other existing issue referencing joining of nodes from control server

- https://github.com/alexellis/k3sup/issues/454
- https://github.com/alexellis/k3sup/issues/385