Azure: Support for custom data during VM creation to support the injection of cloud-init user-data

Azure provides the ability to inject custom user data during VM creation which can then be executed by cloud init to allow automatic VM provisioning following boot. cloud init is a popular way to inject provisioning scripts during VM creation, providing a cleaner alternative to ssh-based provisioning. This functionality existed in azurecompute-arm before the refactoring which replaced the VM deployment template API with the VM api.