Drop bare-metal virt_cluster
    
    Pillar is not used for anything useful. The mine function will be moved to a
    common pillar, as it is referenced in other virt_cluster pillars as well.
    
    The grain is moved to a <cluster>-bare grain to indicate which
    virtualization cluster a machine is responsible for.
    
    Signed-off-by: Georg Pfuetzenreuter <georg.pfuetzenreuter@suse.com>