Allow to change in-memory machine image

[FlorinPeter]

Summary

Currently the machine image (https://github.com/onmetal/onmetal-api/blob/main/api/compute/v1alpha1/machine_types.go#L38) can not be changed after it was set.

Motivation

This behavior currently blocks the update of bare metal machines because the initial image can not be changed and it is not an option to remove the machine assignment and recreate it because some bare metal machines are used for storage pools.

Because this image is used for in-memory booting it should not break something if it could be changed.