Handle sparse image

Many drivers like rbd and filesystem support sparse image, meaning not really write null byte sequences but only the data itself at a given offset, the "holes" who can appear will automatically interpreted by the storage backend as null bytes, and do not really consume your storage. As glance deal with instance image, it appear that they are majorly composed of null bytes sequence to represent the whole disk size of the instances, by exemple for the 8GB base CentOS 7 cloud image : 1GB of data / 7GB of holes, so the storage and upload time improvement will be significant.