create a way to get the cluster context from the cluster

[miekg]

Right now when you have a running cluster and want to use it on another machine you need to copy the uncloud.yaml config over or cut and paste into your local config.

(of course you also need fiddle with ssh keys, but assume that is taken care off - although having a command that upload a key and places it in ~/.ssh/config for the uncloud user could be handy as well)

A good place is likely uc ctx, but uc machine ... already does things with ssh and a lot of things already live there, like other cluster commands.

Proposing to add a uc machine context that adds the cluster context to your local context:

1. -c can be reused, this will be the name of the context, default-N otherwise
2. The [schema://]USER@HOST[:PORT will be reused for a machine connections

Note sure if the machines dns names are actually stored in the cluster, otherwise it will be tricky to get those.

[miekg]

well, we have the public IP, good enough for now, but should be documented.

with the machine ID we can also check if a machine is already included in a context, then we put the rest of them in that same section.