The `image` field in the AzureMachineSpec needs to point to an RHCOS image. For marketplace images, those images should already be available.

For non-marketplace images, we need to create an image for the users, using the VHD from the RHCOS stream.

The image could created in the PreProvision hook: https://github.com/openshift/installer/blob/master/pkg/infrastructure/clusterapi/types.go#L26

Technicaly it could also be done in the InfraAvailable hook, if that is needed.