Support SSL cert auto-rotation using Azure Key Vault

#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](https://azure.microsoft.com/en-us/blog/keep-credentials-out-of-code-introducing-azure-ad-managed-service-identity/), which is currently available as a [VM extension](https://docs.microsoft.com/en-us/azure/active-directory/managed-service-identity/tutorial-linux-vm-access-nonaad) that's being replaced by [IMDS](https://aka.ms/azureimds). 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.