doc: Removed nova-volume, early Ceph references and Folsom references.

fixes: 5006

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Restored show_image_direct and added a link to older versions.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
doc: Removed the Folsom reference.

Signed-off-by: John Wilkins <john.wilkins@inktank.com>
Merge pull request #792 from ceph/wip-doc-openstack

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>