Define cluster images in a single variable

[sjpb]

• Update terraform/ directories to tofu/ to avoid confusion.
• Change skeleton terraform so that cluster images can be defined in a single cluster_image_ids variable (which can be supplied via a single auto-loaded variables file). This will support future changes for slurm-controlled rebuild.
• Swaps CI from Terraform to OpenTofu

Note the changes to the stackhpc terraform are mostly because we need to provide image names as the input - because the same image has different IDs across our CI clouds.

[sjpb]

Closing, don't think this is the right way to go about it.