Allow setting up an alternative container registry for Lokomotive

[iaguis]

Some users would like to use a different container registry (potentially even a local one for security reasons). Lokomotive doesn't support this use case because we hardcode container images in both cluster and components.

We should allow using an alternative container registry. This might involve exposing all container images for components and control plane to the user so they can customize them.

[invidian]

Should we just allow to generically to use different images for core components? E.g. to test custom etcd version easily?

[iaguis]

That sounds like a good idea to me.

[invidian]

Note: we should document, that setting custom images is not officially supported, as they might be set to anything, e.g. busybox:latest.

[invidian]

Because of Docker Hub limits provisioning reliability is now reduced, I think we should prioritize this. See #1225 for more details.