CBQE-953: Topologies for XDCR

- ring
- chain, star (already supported)

Change-Id: I388b73c2225c22c8b6ae784e7978b26ebd71a4ea
Reviewed-on: http://review.couchbase.org/27094
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Ketaki Gangal <ketakigangal@gmail.com>