Make DRBD version queryable from noded

gnt-cluster verify should issue a warning if there are multiple DRBD
versions present in a node group. In order to do so, the DRBD version
has to be queryable from noded.

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