Ceph RBD backend support in glance and cinder

Ceph RBD backend support in glance and cinder
I looked if we can generate a UUID in Ruby but seems like in version 1.8 we cannot. From 1.9 onwards there's `SecureRandom.uuid`, but i think we need stay be compatible with 1.8.7. We can either keep the default an empty string or implement a custom snippet to generate a UUID. I'd lean towards an empty string, i think the user will override it with their own value anyway.