Mapnik renderer configuration not part of the renderer root configuration

- All configuration must be moved into `renderer.mapnik`
 - see `config/environments/*.js.example` for reference

lib/cartodb/server_options.js

@@ -70,6 +70,7 @@ module.exports = function(redisPool) {
             statsInterval: rendererConfig.statsInterval
         },
         renderer: {
+            mapnik: rendererConfig.mapnik,
             http: rendererConfig.http
         },
         redis: global.environment.redis,