anaconda_help: try a wait_still_screen after install_destination
    
    Still hitting a fail sometimes on the spoke after Installation
    Destination, when anaconda is still sorting things out and the
    test tries to do stuff too fast. e.g.
    https://openqa.stg.fedoraproject.org/tests/1206252 . See if this
    helps.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>