better support for openstack/ceph/rbd integration

if you've got a separately deployed ceph/rbd cluster, where you're manually
managing your rbd pools, ueers etc, you can set:

node['ceph']['install_method'] = 'manual'
node['cinder']['storage']['provider'] = 'rbd'

This will drop all the necessary config into the cinder.conf, leaving
you to manage ceph

If you're using chef to also deploy/configure ceph, you can set the
following:

node['ceph']['install_method'] = 'chef'
node['cinder']['storage']['provider'] = 'rbd'

(as well as all the ceph related environment variables).  This will install
ceph packages, drop the ceph config, create the correct user/pool for
cinder to use.