Image Type is Erroneously Required

`image_type` is going away, but until the disk_format and container format stuff merge we have a bit of a problem.

This patch https://code.launchpad.net/~jaypipes/glance/bug704854/+merge/50773 introduced a requirement on image.`type` to always be present.

However, when we take a snapshot from nova, however, we don't know upfront (in the reserve phase) which format the underlying virt-layer is going to use (raw vs vhd).

So, we need to be able to queue an image without a `type` and then add one before it goes into `saving`.

Image.`type` is nullable in the DB, and this pattern was working up until this patch, so I don't think it's a problem to revert back to old behavior with respect to image type. The rest of the patch seems fine.