Resolving issues between repos.yaml and make_spack

The issue

In the current versions of Spack, one can now specify in repos.yaml a git repository and branch for each repository, and let Spack check it out for you. This provides a potential conflict with the make_spack script, which currently checks out a particular branch of the various recipe repositories; where a different branch might be desired for a particular repository in the two places. Thus we do not currently have a single source of truth for recipe repositories.

The proposed solution

So to regain the status of having a single source of truth for the recipe repositories, the following changes are proposed:

• remove the recipe repositories from make_spack's list of things to checkout
• create the repos.yaml, config.yaml, mirrors.yaml, etc. the $SPACK_ROOT/etc/spack/site scope directory, by copying from a template directory in fermi-spack-tools, rather than checking out a repository in /etc/spack.
• use spack repo update to fetch recipe repositories in make_spack