use kv list version of ns_config:set in handle_internal_settings_post

Change-Id: Ic8606b83098d18d1b78c7e83cca233774d0d54e2
Reviewed-on: http://review.couchbase.org/44541
Tested-by: Artem Stemkovski <artem@couchbase.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>