WebUI Tests: changing how the login screen is detected
    
    The "rcue-login-screen" element does not exist anymore. Changing the
    code to use the ".login-pf" instead.
    
    With the change, it's also necessary to check if the login screen is still
    visible when trying to fill the fields of new password, otherwise a
    StaleElementReferenceException exception will be raised.
    
    Reviewed-By: Petr Vobornik <pvoborni@redhat.com>