Added RadosGW support

Originally https://github.com/crowbar/barclamp-ceph/pull/55

Partially ported from upstream cookbook.
> > But this is still wrong: what we care about is keystone being deployed in this SUSE Cloud 
> 
> Really? Before, you wanted to raise an error if keystone is not avialable on this node...

I wanted that as the minimum. What you did here is something in the middle, but it has the downside that if it's not working, people don't know about it. So either tell people it's not working (hence raising an error) or implement it the full way (which is what I described above).