AuthenticationConfig can be queried for logins too (first step).
* bzrlib/transport/ssh.py: (_paramiko_auth): Try to get a user from AuthenticationConfig.
* bzrlib/smtp_connection.py: (SMTPConnection._authenticate): Try to get a user from AuthenticationConfig.
* bzrlib/transport/ftp.py: (FtpTransport._create_connection): Try to get a user from AuthenticationConfig. Credentials are now (user, password) instead of just password.
* bzrlib/tests/test_ftp_transport.py: (TestFTPServerUI._add_authorized_user): Cleanup by refactoring.
* bzrlib/config.py: (AuthenticationConfig.get_user): New method to get logins.