CBQE-0 reduce wait time after rebalance in upgrade test

Change-Id: I7b5e62221337eec370e590f4e70f1f28146f6d70
Reviewed-on: http://review.couchbase.org/40815
Reviewed-by: Thuan Nguyen <soccon@gmail.com>
Tested-by: Thuan Nguyen <soccon@gmail.com>