Ignore offline node errors when removing disks

When we delete DRBD disks from some instance, we do not want to get
errors due to nodes other than that instance's primary being offline.

Signed-off-by: Agata Murawska <agatamurawska@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>
Reviewed-by: René Nussbaumer <rn@google.com>