cache db and index directories in env variable on ns_server

to reduce traffic between ns_server and ns_couchdb nodes

Change-Id: If5475c203e26273fe28d1ccefd7e619c3da6e102
Reviewed-on: http://review.couchbase.org/43435
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>