Embrace HCLv2 variables and rethink recommending go templates

HCL variables already allow for many things which currently are simply ugly in nomad-pack.
See https://github.com/hashicorp/nomad-pack/issues/72#issuecomment-949042509 vs
https://github.com/hashicorp/nomad-pack-community-registry/blob/c4116f1ca28b4a19327c95c45e7bf818564209a6/packs/traefik/templates/traefik.nomad.tpl#L3-L4

So what can we do against that? It imo is okay to use go-templates in cases where HCL does not suffice or for rendering text data (ie outputs.tpl). But given that we already define `variables.hcl` it would be very useful to have those show up in the job templates as hcl variables as well. This way one would not have to rely on go-templates so much.

What do you all think?

Start by reading the linked discussion and comparing variables.hcl with the referenced traefik.nomad.tpl lines. Review how job templates and outputs.tpl currently use Go templates, then determine the intended boundary between HCL variables and Go templates. Done should include an agreed approach for the template behavior.

Written by the indexing model from the issue text.