CBQE-928: fix online_upgrade_incremental

Change-Id: I7ab81a039b0752409b736920d304eb50cdaf7a97
Reviewed-on: http://review.couchbase.org/24137
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>