Use Iron Bank images for K3s

[RothAndrew]

Currently we use images for K3s from GitHub. Our goal is to be all Iron Bank for compliance reasons. The K3s images that we get from https://github.com/k3s-io/k3s/releases/download/<InsertVersionHere>/k3s-airgap-images-amd64.tar.zst should be added to Iron Bank and pulled from there instead.

[jeff-mccoy]

Closing this for now, will re-evaluate all Iron Bank requirements for the main init package later.