MB-100: refactor P0 rebalance tests

Change-Id: I78ef4aadb5cfe77b970833bdb6bdfcdcbf8ed265
Reviewed-on: http://review.couchbase.org/25706
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Andrei Baranouski <andrei.baranouski@gmail.com>