Make UEFI testing the default, add Secure Boot testing
    
    This essentially inverts the x86_64 machines so that '64bit' is
    UEFI and instead of a variant 'uefi' machine we have a variant
    'bios' machine that is BIOS. The point is to make UEFI testing
    the default. We also enable Secure Boot in the UEFI testing,
    and add a test of UEFI fallback booting on various products.
    
    Signed-off-by: Adam Williamson <awilliam@redhat.com>