I can confirm this working on a 1.8 kubernetes cluster

apiVersion: v1
kind: PersistentVolumeClaim
metadata:
  name: mongo-data
spec:
  accessModes:
  - ReadWriteOnce
  resources:
    requests:
      storage: 1Gi
---
apiVersion: apps/v1beta1
kind: Deployment
metadata:
  name: mongo
spec:
  replicas: 1
  template:
    metadata:
      labels:
         app: mongo
    spec:
      containers:
      - name: mongo
        image: clutteredcode/mongo-alpine:latest
        imagePullPolicy: Always
        ports:
        - containerPort: 27017
        volumeMounts:
        - name: mongodata
          mountPath: /data/db
      volumes:
      - name: mongodata
        persistentVolumeClaim:
          claimName: mongo-data
---
apiVersion: v1
kind: Service
metadata:
  name: mongo
  labels:
    app: mongo
spec:
  selector:
    app: mongo
  ports:
  - port: 27017
    targetPort: 27017

# kubectl create -f mongodb.yaml

apiVersion: extensions/v1beta1
kind: Deployment
metadata:
  name: rocketchat
spec:
  replicas: 1
  template:
    metadata:
      labels:
         app: rocketchat
    spec:
      containers:
      - name: rocketchat
        image: rocketchat/rocket.chat:latest
        imagePullPolicy: Always
        ports:
        - containerPort: 3000
        env:
        - name: ROOT_URL
          value: "http://chat.testbed.lan"
        - name: MONGO_URL
          value: "mongodb://rocket:[email protected]:27017/rocketchat"
---
apiVersion: v1
kind: Service
metadata:
  name: rocketchat
  labels:
    app: rocketchat
spec:
  selector:
    app: rocketchat
  type: NodePort
  ports:
  - port: 3000
    targetPort: 3000

# kubectl create -f rocket.yaml