Support Datadog universal service tagging

[brettcurtis]

To get the full range of unified service tagging when using Kubernetes, add environment variables to both the deployment object level and the pod template spec level:

apiVersion: apps/v1
kind: Deployment
metadata:
  labels:
    tags.datadoghq.com/env: "<ENV>"
    tags.datadoghq.com/service: "<SERVICE>"
    tags.datadoghq.com/version: "<VERSION>" 
...
template:
  metadata:
    labels:
      tags.datadoghq.com/env: "<ENV>"
      tags.datadoghq.com/service: "<SERVICE>"
      tags.datadoghq.com/version: "<VERSION>" 
  containers:
  -  ...
     env:
          - name: DD_ENV
            valueFrom:
              fieldRef:
                fieldPath: metadata.labels['tags.datadoghq.com/env']
          - name: DD_SERVICE
            valueFrom:
              fieldRef:
                fieldPath: metadata.labels['tags.datadoghq.com/service']
          - name: DD_VERSION 
            valueFrom: 
              fieldRef: 
                fieldPath: metadata.labels['tags.datadoghq.com/version']