LUNodeAdd: Verify version in Prereq

There are other ways to leave the cluster in a broken state than just
the version check. However they are not very trivial to fix in 2.5. So
leave it up to 2.6 for a nicer fix.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>