CBES-16: fix for es topology test

It's not safe to assume that src_master and src_nodes[0] are the same.
this causes verification to fail in topology tests

Change-Id: I6827400a5f36e74d5c37149f66ddaf22f51efa2b
Reviewed-on: http://review.couchbase.org/29307
Reviewed-by: Tommie McAfee <tommie@couchbase.com>
Tested-by: Tommie McAfee <tommie@couchbase.com>