Removes image type validation in the Glance registry.
Removes image type validation in the Glance registry.

Nova xs-unified-images requires adding a new image type 'vhd' along side of 'machine', 'raw', etc. Since image_type isn't used by Glance itself (it merely stores the value and hands it back to Nova), Glance shouldn't be the arbiter of what values are acceptable-- Nova should. This promotes loose-coupling and ensures that Glance won't have to change in lock-step with Nova.

More to the point, this a stop-gap to make Glance work with the new xs-unified-images branch until we get proper `container_format` and `disk_format` modeling in Glance. When that happens, we may end up ditching `image_type` altogether.
openstack/glance,ozamiatin/glance,stevelle/glance,takeshineshiro/glance,ntt-sic/glance,darren-wang/gl,sigmavirus24/glance,wkoathp/glance,JioCloud/glance,redhat-openstack/glance,stevelle/glance,vuntz/glance,tanglei528/glance,klmitch/glance,citrix-openstack-build/glance,vuntz/glance,SUSE-Cloud/glance,rickerc/glance_audit,redhat-openstack/glance,takeshineshiro/glance,rajalokan/glance,jumpstarter-io/glance,darren-wang/gl,kfwang/Glance-OVA-OVF,openstack/glance,tanglei528/glance,scripnichenko/glance,ozamiatin/glance,paramite/glance,SUSE-Cloud/glance,akash1808/glance,dims/glance,jumpstarter-io/glance,rajalokan/glance,dims/glance,rickerc/glance_audit,cloudbau/glance,akash1808/glance,cloudbau/glance,JioCloud/glance,saeki-masaki/glance,klmitch/glance,scripnichenko/glance,wkoathp/glance,ntt-sic/glance,saeki-masaki/glance,citrix-openstack-build/glance,paramite/glance,openstack/glance,kfwang/Glance-OVA-OVF,sigmavirus24/glance