CBQE-0: append instead of extend in incremental_rebalance test

Change-Id: I6d8e176c0d8fe86542dde8459613980d30bb1d50
Reviewed-on: http://review.couchbase.org/32909
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: Andrei Baranouski <andrei.baranouski@gmail.com>