Make certificate renewal a task

src/tasks.js

@@ -15,7 +15,8 @@ exports = module.exports = {
     // task types. if you add a task here, fill up the function table in taskworker
     TASK_BACKUP: 'backup',
     TASK_UPDATE: 'update',
-    TASK_MIGRATE: 'migrate'
+    TASK_MIGRATE: 'migrate',
+    TASK_RENEW_CERTS: 'renewcerts'
 };
 
 let assert = require('assert'),