Support PPAs on cluster machines

[jdreed]

We need a trivial way to enable PPAs on cluster machines, as academic software is increasingly being distributed this way (Julia, OpenFOAM). My plan for this basically consists of automating the adding of a PPA, and configuring pinning in a clever way such that you only get whitelisted packages from that PPA (so we don't lose if some PPA starts packaging, e.g. bash). This would require another clause in the auto-updater, I think, since machines should continue to be functional if the PPA is down, so we don't want to add PPA packages to the metapackages.