Correct sort_key values for Images API v2
Correct sort_key values for Images API v2

Current docs say that any image attribute can be used as a sort key,
but the code restricts the allowed values.

Change-Id: Ib4be4be143095c869f91934372a9e1f3464865ea
Closes-bug: #1482328

Tesora/tesora-api-site,Tesora/tesora-api-site,Tesora/tesora-api-site,dreamhost/api-site,dreamhost/api-site,Tesora/tesora-api-site,dreamhost/api-site,dreamhost/api-site,openstack/api-site,openstack/api-site,dreamhost/api-site,Tesora/tesora-api-site,dreamhost/api-site,openstack/api-site,dreamhost/api-site,openstack/api-site,Tesora/tesora-api-site,Tesora/tesora-api-site