[capi] Building images using the k8s ci artifacts fails

[detiber]

The list of kubernetes_imgs that we have in images/capi/ansible/roles/kubernetes/defaults.main.yml includes pause.tar, coredns.tar, and etcd.tar, but they are not included in the k8s ci artifacts (since they are built, versioned and distributed separately).

If we want to automate loading of these images, we should likely use a separate variables for their source, type, versions rather than overloading the use of kubernetes_http_source, kubernetes_semver, and kubernetes_source_type

[detiber]

This is causing issues with attempting to use the k8s ci artifacts for building an image here: kubernetes-sigs/cluster-api-provider-gcp#292

/assign @codenrhoden

[codenrhoden]

Discussed this topic with @detiber today.

He's going to try out adding a flag that, even when setting kubernetes_source_type = http, allows you to request that the images not associated with a specific K8s release ('pause', 'coredns', 'etcd') not be pre-loaded. An image built in this way would fall-back to pulling those images on-demand.