CBQE-2241: upgrade test fix

fix servers amount involved in rebalance

Change-Id: Ic21d5536496c9aace796033a0481d6f1531fe256
Reviewed-on: http://review.couchbase.org/36888
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>
Tested-by: buildbot <build@couchbase.com>