CBQE-928: online upgrade test fix

Change-Id: I8f24ecbb896539370bb8d18f4dbfe79337f69462
Reviewed-on: http://review.couchbase.org/24181
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>