Added RadosGW support

Originally https://github.com/crowbar/barclamp-ceph/pull/55

Partially ported from upstream cookbook.

Depends on https://github.com/crowbar/barclamp-keystone/pull/237
We need add this to `validate_proposal_after_save`
```
    unless proposal["deployment"]["ceph"]["elements"]["ceph-radosgw"].empty?
      keystone_dependency = false
      ProposalObject.find_proposals("keystone").each {|p|
        if (p.status == "ready") || (p.status == "pending")
          keystone_dependency = true
        end
      }
      validation_error("Keystone wasn't deployed. Ceph with RadosGW requires Keystone.") if keystone_error
    end
```