images: fix wrong display after deleting all images in a namespace

After deleting all cloudius images through 'capstan rmi', capstan images
will still show that there is a image named 'cloudius'. We found that
the 'cloudius' namespace is treated as a normal image that has no
namespace.

Signed-off-by: wangbicheng <wangbicheng@huawei.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>