rewrite leap_cli ssh code

We want to support 25519 ssh keys. We are using old version of capistrano that requires an old version of net:ssh.

I need to rewrite how leap_cli handles ssh so that we can use the bleeding edge net:ssh so that we can use 25519.

Better ssh code will also make the code better and remove the stupid dependency on capistrano.

(from redmine: created on 2015-01-06, closed on 2016-09-07, relates #7591 (closed), duplicates #8002 (closed))