Add pip to analysis releases, 1.0 branch (2017.11.28.)

While trying to make cookiecutter work on lxplus, I finally decided to just take a deep breath, and finally provide a functional pip installation as part of the analysis releases. Since analysers using Python have been asking for this for quite a while now.

I tested that with the update I was able to use pip for some basic installs on all of our regular installation platforms. Though I was probably not as thorough with this as I could've been...

@lheinric, this is how I'd solve the problem of using cookiecutter on lxplus. @will, you should know about this as well, as I'm modifying AthAnalysisExternals slightly as well. Finally, @ksuruliz should probably also see this change.