Fix group verification of offline nodes

Commit aef59ae7 reworked the file verification, but forgot to take
into account offline nodes.

The fact that this was not detected yet is due to the fact that we
don't test clusters with offline nodes in QA :(

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>