Issue title: Add an e2e test that uses pod identity via keyvault flexvol
 Issue body: **Describe the request**

**Explain why AKS Engine needs it**

**Describe the solution you'd like**

**Describe alternatives you've considered**

**Additional context**

 Comments: 
 Comment 0: https://github.com/Azure/aks-engine/blob/master/examples/addons/aad-pod-identity/README.md
 Comment 1: This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

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