Make sure we have keep-alive probes for SSH

@ppp0 @tomaszdurka 
Got this after a long running SSH call in capistrano:
```
connection closed by remote host (Net::SSH::Disconnect)
```

Maybe we don't have keepalive SSH probes enabled?
On debian servers and client and OSX clients.
See http://railsadventures.wordpress.com/2013/05/03/ssh-connection-closed-while-deploying-with-capistrano/