Check for uniform DRBD version in node group

This check is performed during gnt-cluster verify and outputs warnings
if at least two different DRBD versions are found within a node group.
In such a case, all nodes with their installed DRBD version are
displayed.

Signed-off-by: Thomas Thrainer <thomasth@google.com>
Reviewed-by: Helga Velroyen <helgav@google.com>