Terminal Image configuration per infrastructure

[petersutter]

What would you like to be added: Add the possibility to configure images for the terminal per infrastructure and with the matching kubectl version.

Why is this needed: This is needed to configure infrastructure specific images (e.g. containing the cli of the respective infrastructure)

[vlerenc]

There is another angle/dimension: the kubectl version and whether or not it fits the shoot cluster Kubernetes version.

The skew policy https://kubernetes.io/docs/setup/release/version-skew-policy/#kubectl means we would have to pick the right image with the right kubectl version per cluster, i.e. we should use a toolbelt with a kubectl in v1.15 for a shoot cluster in v1.15 (more skew is allowed, but since it isn't fully backward-compatible, we anyways have to pick a version within the "skew range", but then we must have the images anyway and can pick the matching version).

Could you please add that as well to this ticket? I also created gardener/ops-toolbelt#37.