MB-100 slight ns_rebalancer re-factoring

for better readability

Change-Id: I79dfca879aca6ef7543e3e537493b7367d9e41d7
Reviewed-on: http://review.couchbase.org/33506
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>