Cannot access with localhost:27017

Hi there, I meet some problem that access to the mongodb service

1. openfaas-db-mongodb successful running on the k8s cluter
2. and my function (based on node8-express-template) cannot connect to mongodb
3. logs are:
```shell
2018/07/17 17:53:14 stderr: (node:7) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 1): MongoNetworkError: failed to connect to server [127.0.0.1:27017] on first connect [MongoNetworkError: connect ECONNREFUSED 127.0.0.1:27017]
```