rbd: Use cloning for deploying templates instead of a copy

RBD format 2 supports cloning (aka layering) where one base image can serve
as a parent image for multiple child images.

This enables fast deployment of a large amount of virtual machines, but it also
saves spaces on the Ceph cluster and improves performance due to better caching.

Qemu-img doesn't support RBD format 2 (yet), so to enable these functions the
RADOS/RBD Java bindings are required.