The Ceph RBD deployment guide for OpenStack [1] suggests use of
hw_disk_discard from Juno. This change adds support into the module
for such a config option.
1. http://docs.ceph.com/docs/master/rbd/rbd-openstack/
Change-Id: Ib0198d63d7e1de4677a1345da1f710277a63d13b