Centos7 packages

[scottwittenburg]

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[scottwittenburg]

@tgamblin @becker33 Here's a change, based on top of #4, as an attempt to validate a centos7-based tutorial. I just assumed gcc@7.2.1 would be available on the tutorial base image (amazonlinux), but had to bootstrap it here.

I also assumed the tutorial base image would have gcc@4.8.5 installed, but if that's incorrect, I can add it here as just a regular package so it gets added to the mirror. I'm using spack/centos7 to build this pipeline, and it has gcc@4.8.5 as it's system compiler.