[rel-2.5.1] point ns_server to branch for-rackaware

Change-Id: I5edc734c5c851ad30c0b16f0bf939e308df9598f
Reviewed-on: http://review.couchbase.org/33517
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>