subctl: provide a connect operation combining deploy-broker and join

[skitt]

What would you like to be added:

A subctl connect operation which connects two or more clusters using Submariner.

Why is this needed:

For small numbers of clusters, having a separate deploy-broker operation producing a file that needs to be provided when joining is cumbersome. On top of that, semantically, there’s an ambiguity with join — it joins a cluster to a clusterset (or put another way, to a broker), but users may want to “join” clusters that will communicate with each other.

Having access to all the clusters also means that some parameters can be auto-configured, or at least checked.