Disable hidden grub menu for uploaded base installs
    
    At least one test (desktop_notifications_postinstall) boots from
    the disk image uploaded by install_default_upload, and needs to
    access the grub menu. On F29+ Workstation this is failing,
    because the grub menu is now hidden by default, so when the test
    boots, it never sees the bootloader screen, and fails.
    
    I considered trying to teach it to hold down shift or hit f8 or
    esc at the right time, but that seems like it might be hard. So
    instead let's just try to disable the hidden menu when we're
    about to upload the installed system image. This is kinda going
    against the 'preserve natural system behaviour' principle we try
    to use for openQA, but I think it's OK as we do have other tests
    that will exercise the 'hidden boot menu' stuff to some extent.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>