Specify image size to avoid backend resize

When uploading a volume to glance as an image, the glance server don't know the image size, so the backend storage server(such as ceph) need to resize the image every time it received new chunk of data(by default 64K). So there will be huge times of resize operations that will terribly impact the performance. This BP aims to tell the image size to glance server at the initial phase of uploading, so that glance backend such as ceph can allocate an image with exact size, and don't need to resize the image again and again