support list conference (#321)

* support list conference

* add test case

* fix conference action requires tag

* fix failing test case

lib/helpers/realtimedb-helpers.js

@@ -13,6 +13,7 @@ const {
   deleteKey,
   incrKey,
   client: redisClient,
+  listConferences
 } = require('@jambonz/realtimedb-helpers')({}, logger);
 
 module.exports = {
@@ -27,5 +28,6 @@ module.exports = {
   retrieveKey,
   deleteKey,
   redisClient,
-  incrKey
+  incrKey,
+  listConferences
 };