grubby-bls: lookup default entry by either id or title on grub2
    
    There is a grub2-set-default script that can be used to set the default
    entry. It can take an entry id, title or index as a parameter so grubby
    should be able to also lookup a default entry using the title on grub2.
    
    Related: rhbz#1654936
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>