WIP: ceph config tweaks

http://202.194.15.195/openstack-doc/openstack-compute/admin/content/configuring-hypervisors.html

^ From reading that doc it seems that when setting libvirt_inject_partition to -2, the libvirt_inject_key/password variables will have no effect anyway (any injection will be always disabled).

I'm also a bit puzzled whether we actually need to disable this when using Ceph. I found that the Ceph docs mention it as a good practice (http://ceph.com/docs/next/rbd/rbd-openstack/#configuring-nova), but afaict disabling it will prevent Nova from setting credentials when launching images which do not have cloud-init installed. Maybe in real use cases this is not an issue, but it might be worth verifying with someone so that we make this decision consciously.

Otherwise the PR looks good to me :)