make sure that add_node test correctly works with cluster_run env

Change-Id: I5a1267426c2ec79dab29707fae1509afba74f17c
Reviewed-on: http://review.couchbase.org/37493
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>