More image version fixes and changes.
1. Fixes to allow specific versions of images to be exported; the existing path was reverting back to the highest numbered version. So far this has not come up, but will with APT and Cloud. 2. Add version argument to image_metadata.php, mostly as a convenience. So rather then using the version specific uuid, you can use the image uuid, with a version argument. This is actually more sensible, except for one important fact; it is not possible to locate a deleted image this way, since the image descriptor is gone (only the version descriptors are in the DB). But I went ahead and did it cause there is still some question as to whether we care about being able to export a deleted image. We do not expose these URLs at this time, but you can use one.