Define architecture for doing both el8 and el9 builds.

[ndp-opendap]

We have, for the most part, separated the component tagging in our S3 bucket so it's easy to identify el8 vs el9 assists.

In my el9 branch on this repo there are two "build recipe" files, one for el8 and one for el9. The TravisCI (.travis.yml) has a hard coded definition on which to use:

    - BUILD_RECIPE="el9-build-recipe"

That's fine for now but when we merge this all down to the main branch we need to have a way to detect which version of RHEL is being target by the most recent commit.