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
Method `proposal_dependencies` works only during creating proposal and you are checking here `ceph-radosgw` elements which will be added in `create_proposal` after `proposal_dependencies`