convert image to raw directly in rbd

Currently, boot a vm from image in rbd, if the format of image is not raw, it will be downloaded from rbd and converted in local storage and then it can be uploaded to the rbd. That will take a long time for downloading and uploading for big images. Images can be converteed in rbd directly with rbd's command and 'qemu-image convert xxx', after that can be uploaded directly with glance image--creat --location. But this is complicated， so bp suggest that image can be converted dircely in rbd for nova rbd backend.