Key-based authentication should not keep prompting for SSH password

[dwgnr]

When key-based SSH authentication is enabled via ssh_key_path, the app shouldn't keep prompting for passwords. It knows that the key pair should be used and it's very confusing for the user when the password prompt keeps appearing (even though you can simply press enter).

Ideally, the username should also be cached. I guess it's save to assume that the username is the same across multiple connections. I'm thinking setting the default value to the username last entered. So the user can decide to simply hit enter or to change the username in case it's different on a specific node.