purge-single-property-one-time

Currently， we only can purge all props of image by execute "glance image-update --purge-props imageId".It's not like purge props from flavor , we can't purge single property one time.

Use Case:
There are many props (example: p1, p2, p3, p4 )in image A.As a adminisrator , when i want to purge the p4 from the image. Current we need purge all props first , then update other props(p1, p2, p3) to the image again. We must to improve this bad situation in order to improve usability.