allow changing tensorflow cocoBase from plugin's conf.json

+ default tensorflow plugin is now lite_mobilenet_v2 instead of mobilenet_v2

plugins/tensorflow/ObjectDetectors.js

@@ -23,7 +23,7 @@ module.exports = function(config){
 
   async function loadCocoSsdModal() {
       const modal = await cocossd.load({
-          base: 'mobilenet_v2'
+          base: config.cocoBase || 'lite_mobilenet_v2' //lite_mobilenet_v2
       })
       return modal;
   }