Support SSL cert auto-rotation using Azure Key Vault

[hosungsmsft]

#14 addresses the SSL cert issue, but it also talks about auto-rotating the deployed cert when it's near expiry or revoked. This requires a full integration of Azure MSI, which is currently available as a VM extension that's being replaced by IMDS. Therefore, we better wait for the IMDS MSI support before implementing the cert auto-rotation. Besides, currently the Key Vault needs to be configured (to allow access from VM) after the VM is deployed, so the auto-rotation must be configured after the deployment, which makes this task separate from the template improvement. Therefore, setting up a separate issue here.