Allow to configure server#scp and server#ssh on the instance level.

```
server.ssh_options = {user: 'root'}
server.scp('/etc/passwd', 'bar')
server.ssh('ls -la')
```
Fixes issue where Net::SSH would only use the "password" authentication method

I've already seen that in Capistrano. Not sure exactly where the heuristics in
Net::SSH is failing but this should be fairly save.