doc: Update for OpenStack Juno.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge pull request #2854 from ceph/wip-doc-openstack-juno

doc: Update for OpenStack Juno.

New users working with Nova (Juno) noted that libvirt settings are now under a [libvirt] section, and truncate the leading libvirt_. Made subsections for Havana and Icehouse, added a new subsection for Juno.