Create Vocabulary for Client Variables versus Allocation Variables

Need to create and socialize vocabulary around the interpolatable variables to alloc-runner/template environment about the client machine (`${attr.kernel.name}`)versus the Nomad environment variables that are passed into the allocation at runtime (the `$NOMAD_` variables you can see with the `env` command).

This issue is to track a resolution to [@schmichael's comment](https://github.com/hashicorp/nomad/pull/10807/files#r658286781) in #10807.

Start by reviewing PR #10807 and the linked comment, then compare the alloc-runner/template variables such as `${attr.kernel.name}` with the `$NOMAD_` variables visible through the `env` command. Done means the project has an agreed vocabulary for these two variable categories and that terminology has been socialized.

Written by the indexing model from the issue text.