pass log file as argument to task worker

initially, i thought i can hardcode the log file into taskworker.js
depending on the task type but for apptask, it's not easy to get the
appId from the taskId unless we introspect task arguments as well.
it's easier for now to pass it as an argument.

src/test/apptask-test.js

@@ -5,8 +5,7 @@
 
 'use strict';
 
-var addons = require('../addons.js'),
-    appdb = require('../appdb.js'),
+var appdb = require('../appdb.js'),
     apps = require('../apps.js'),
     apptask = require('../apptask.js'),
     async = require('async'),