Adding athgeneration container setup

This is the first import of the Dockerfile and scripts necessary for building a stand-alone athgeneration container. For the most part this follows what's done for other athena releases. There is a little extra work to pull in some of the repositories for job options and MG5_aMC models, and also to pull in the PDFs that are required for standard jobs. This of course makes the image bigger than we'd like, but it ensures that it does not come with any external dependencies.

Seems to be working for the tutorial jobs: https://atlassoftwaredocs.web.cern.ch/AnalysisSWTutorial/mc_generation/

I don't love faking some of the cvmfs structure, but quite a bit of it is hard-coded in the generation python layer, so I don't see a way around it for old releases. For future releases of course we can work on improving the relocatability of some of these things.