Ignore grub2-install errors if they are expected
    
    With the inclusion of
    https://github.com/rhboot/grub2/commit/a1be2d182af27afb6db0e7fcf9d5ebf40086b2f6
    grub2-install refuses to run in efi environments, making
    appliance-create fail erroneously. The approach here is to skip this
    error if expected, while still calling grub2-install to try to keep the
    functionality as close as possible with older versions
    
    Signed-off-by: Pablo Greco <pgreco@centosproject.org>