Refactor the join command to have tested SSH commands

## Expected Behaviour

We should refactor the join command to have tested SSH commands just like I did for the install command in this patch: https://github.com/alexellis/k3sup/commit/fd18d772401e5e1f895eb42c1bbd8c6f6e2ca260#diff-0ad45407eb1c7c8830d260d38994acb374ec5b7e36bc891e54406206074e5a99

By refactoring to a method, testing becomes easier and regression is less likely between changes and adding additional features such as #274.

## Current Behaviour

String concatenation with fmt.Sprintf, no regression tests

## Possible Solution

Copy the approach of `makeInstallExec` but for `makeJoinExec` perhaps?

Note that there are TWO ssh commands here - one for joining an agent, and one for joining an agent that acts as a backup / secondary server.

See also: join command - https://github.com/alexellis/k3sup/blob/master/cmd/join.go#L205