Move the values for the TOKENS into a secrets file

[robertjahn]

Move these values that are now in notification-service.yaml into a k8 secrets file will make them more secure. So change code and instructions in readme.

        env:
        - name: TEAMS_URL
          value: ""
        - name: BRIDGE_URL
          value: ""
        - name: SLACK_URL
          value: ""
        - name: WEBEXTEAMS_URL
          value: ""

Plus you can have the benefit of deploying the service without having to clone the repo, for example kubectl apply -f https://raw.githubusercontent.com/keptn-contrib/notification-service/master/deploy/notification-service.yaml