Try and fix Cockpit tests breaking with update ISO change
    
    We drop the line for the update ISO from /etc/fstab before
    uploading the image after the cockpit_default test, but we don't
    make sure it's set up again before Cockpit tries to use it, in
    the subsequent Cockpit tests. I don't know why this didn't fail
    on stg before, but it sure as hell is failing in prod...
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>