Added default value for kafka aggregate topic

Author: callmekatootie

config/default.js

@@ -20,7 +20,7 @@ module.exports = {
   },
   BUSAPI_URL: process.env.BUSAPI_URL || 'https://api.topcoder-dev.com/v5',
   KAFKA_ERROR_TOPIC: process.env.KAFKA_ERROR_TOPIC || 'error.notification',
-  KAFKA_AGGREGATE_TOPIC: process.env.KAFKA_AGGREGATE_TOPIC,
+  KAFKA_AGGREGATE_TOPIC: process.env.KAFKA_AGGREGATE_TOPIC || 'submission.notification.aggregate',
   CHALLENGEAPI_URL: process.env.CHALLENGEAPI_URL || 'https://api.topcoder-dev.com/v4/challenges',
   AUTH0_URL: process.env.AUTH0_URL, // Auth0 credentials for Submission Service
   AUTH0_AUDIENCE: process.env.AUTH0_AUDIENCE || 'https://www.topcoder.com',