Added RadosGW support

Originally https://github.com/crowbar/barclamp-ceph/pull/55

Partially ported from upstream cookbook.
> Well, but it does not hurt anything.

It's an edge case, but if there's no node with radosgw role, and the keystone proposal is created but not applied, right now, this method will say that the ceph proposal cannot be applied. That's the small bug that I'd like to see fixed.

> And do I know the current node in this function? I can't get that info from 'role' argument, can I?

I think you can get the deployment attributes from `role.override_attributes[@bc_name]`