Fix incompatible with old image without flags

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
cls_rbd_client: Add tests for set/get flags

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>
test_cls_rbd: add tests for cls_rbd image index

Signed-off-by: Haomai Wang <haomaiwang@gmail.com>