Implement --ignore-size in activate-disks

This patch modified OpActivateDisks, LUActivateDisks and gnt-instance
activate-disks to support and pass this option to
_AssembleInstanceDisks.

The patch is quite trivial I think; there should be no issues from it
except if used when not needed.

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