CBQE-928: add online upgrade test with rebalance

Change-Id: Ife6c4d30262dd42279e35a73e3eb46f341cb0cae
Reviewed-on: http://review.couchbase.org/24099
Reviewed-by: Iryna Mironava <irynamironava@yandex.ru>
Tested-by: Iryna Mironava <irynamironava@yandex.ru>