Export a Direct URL For Upload

Just as a client is can look up a direct URL location for a more efficient download, a client should be able to request a direct_url for upload.

Take for example the case where nova-compute would like to snapshot an image. It wishes to upload this image to glance, but it would prefer to speak directly to the backend store instead of sending all of the image bits first through glance. However, nova does not want to be concerned with all of the storage points that glance abstracts, nor how. nova simply wants to upload it as a new image ID. To accomplish both things, nova can ask glance for a direct upload URL for use with the new image.