CUDA installations

[Neves-P]

If one follows a regular build process for CUDA (i.e., add a CUDA easyconfig file to an easystack file), the resulting installation will predictably end up in /cvmfs/dev.eessi.io/projectname/... (see #32).

Ideally we want to be able to deploy CUDA to a more centralized location, and manage those installations ourselves. Hence, there needs to be a way to have CUDA (and other similar scenarios) land somewhere "central" to the CVMFS repository that can be easily accessed by developers using dev.eessi.io and so that they can build against it.

[ocaisa]

This requires a change to the hooks so that the symlinks are also triggered for builds under dev.eessi.io. We would also need some documentation about how to unbreak these symlinks (our scripts right now do not allow you to tweak the CUDA versions to install)