Add a tool for cleaning up clusters

Add a tool for cleaning up clusters

This tool (that is not installed, just available in the source tree)
helps with cleaning up clusters to a (as much as possible) pristine
state. This helps in preparing for QA runs. By cleaning I mean removing
all instances, logical volumes, nodes, etc. - so this will cause
complete data-loss if it's run on a normal cluster.

Limitations: it only deals with drbd8 and not md (if any md devices are
in use which use drbd devices, they will not be cleaned up). Also the
logic behind the cleaning up is not bulet-proof.

Reviewed-by: ultrotter