CBQE-0: fix add_nodes_with_ddoc_ops test

get stats only from existing cluster nodes

Change-Id: Id00a2957498816d29a6b8363f332ddc65b370c48
Reviewed-on: http://review.couchbase.org/32419
Tested-by: Iryna Mironava <irynamironava@yandex.ru>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>