Update docs to reflect exposure of image 'owner' in v2.

The image 'owner' property was exposed by
https://review.openstack.org/#/c/69887/ .
This updates the docs to show that.

Change-Id: I341acf5384a7c37effc9a6d14073cd4a82307696
Closes-bug: #1277344
Merge "Update docs to reflect exposure of image 'owner' in v2."