Addng configuration option ``rbd_thin_provisioning`` and
``filesystem_thin_provisioning`` to rbd and filesystem
backends, to enable or not sparse upload.
A sparse file means that we do not actually write null byte
sequences but only the data itself at a given offset, the
"holes" which can appear will automatically be interpreted by
the storage backend as null bytes, and do not really consume
your storage.
Partially Implements: blueprint handle-sparse-image
Change-Id: Ic95fa45af0f1db92d8425862c6267f466764fbbe
da9e1a7f4e