Recreate instance disks: allow changing nodes

This patch introduces the option of changing an instance's nodes when
doing the disk recreation. The rationale is that currently if an
instance lives on a node that has gone down and is marked offline,
it's not possible to re-create the disks and reinstall the instance on
a different node without hacking the config file.

Additionally, the LU now locks the instance's nodes (which was not
done before), as we most likely allocate new resources on them.

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