crush: update choose_args when items are added/removed

crush: update choose_args when items are added/removed

When items are added:

- the value in the weight set is set to the target weight. It is assumed
  that weight set are updated on a regular basis and will eventually be
  set to a value that prevents excessive over/under filling.

- the value in the id list is set to the item id.

When items are removed, their weight / ids in all choose_args are
removed.

Signed-off-by: Loic Dachary <loic@dachary.org>