x2go/{backends/control/plain,sshproxy}.py: Don't let look_for_keys override...
x2go/{backends/control/plain,sshproxy}.py: Don't let look_for_keys override specified private key. The specified private key should win.
x2go/{backends/control/plain,sshproxy}.py: Don't let look_for_keys override specified private key. The specified private key should win.