This change makes it easier/lazier for people to configure their kernel
and os images. If users do not care about the specifics, and do not need
to bring custom images, they can simply set the following:

In the manifest:

---
apiVersion: infrastructure.cluster.x-k8s.io/v1alpha1
kind: MicrovmMachineTemplate
spec:
  template:
    spec:
      kernelVersion: "${KERNEL_VERSION}"
      osVersion: "${OS_VERSION}"
      ...

Via the command line:

export KERNEL_VERSION=5.10.77
export OS_VERSION="${KUBERNETES_VERSION#*v}" # they already set this anyway

And we work out the rest. Long form, this is:

---
apiVersion: infrastructure.cluster.x-k8s.io/v1alpha1
kind: MicrovmMachineTemplate
spec:
  template:
    spec:
      rootVolume:
        id: root
        image: "${MVM_ROOT_IMAGE:=ghcr.io/weaveworks-liquidmetal/capmvm-kubernetes:1.23.5}"
      kernel:
        filename: "boot/vmlinux"
        image: "${MVM_KERNEL_IMAGE:=ghcr.io/weaveworks-liquidmetal/kernel-bin:5.10.77}"
      additionalVolumes:
      - id: modules
        image: "${MVM_KERNEL_MODULES_IMAGE:=ghcr.io/weaveworks-liquidmetal/kernel-modules:5.10.77}"
        mountPoint: /lib/modules

Or via env vars:

export MVM_ROOT_IMAGE=docker.io/richardcase/ubuntu-bionic-test:cloudimage_v0.0.1
export MVM_KERNEL_IMAGE=docker.io/richardcase/ubuntu-bionic-kernel:0.0.11
export MVM_KERNEL_MODULES_IMAGE=ghcr.io/weaveworks-liquidmetal/kernel-modules:5.10.77

The previously existing fields will continue to exist and can be used if desired.

If both the short form options and the long-form fields are set, the
short-form values are what will be used. (Or I can make it the long-form
whichever people prefer.)

The previously required kernel and root volume fields are now optional so, the machine webhook has been updated to ensure that at least one of the options are provided.

Part of liquidmetal-dev/image-builder#49

Depends on liquidmetal-dev/controller-pkg#3 and liquidmetal-dev/controller-pkg#2 requiring a mod bump here after those 2 are merged.