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>