MB-100: collect utilization stats after rebalance in perf. tests

Reset stats right before rebalance

Change-Id: I9ecde8b2528af4a601381ee68fb48c240eb1a5dc
Reviewed-on: http://review.couchbase.org/24103
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Pavel Paulau <pavel.paulau@gmail.com>