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.

glance/registry/db/models.py

@@ -103,7 +103,7 @@ class Image(BASE, ModelBase):
 
     @validates('type')
     def validate_type(self, key, type):
-        if not type in ('machine', 'kernel', 'ramdisk', 'raw'):
+        if not type in ('machine', 'kernel', 'ramdisk', 'raw', 'vhd'):
             raise exception.Invalid(
                 "Invalid image type '%s' for image." % type)
         return type