Fix LUInstanceRecreateDisks with iallocator

Fix LUInstanceRecreateDisks with iallocator

Recent iallocator changes (commit fb60bc6a, “iallocator: Add node
whitelist”) broke the parameter validation for LUInstanceRecreateDisks
(the other LUs were manually fixed already):

  $ gnt-instance recreate-disks -I hail --dry-run instance3
  Failure: prerequisites not met for this operation:
  error type: wrong_input, error details:
  Request is missing 'node_whitelist' parameter

By just defining the new parameter, the LU works normally (“INFO:
Selected nodes for instance instance3 via iallocator hail: node2”).

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>