Add ability to create venv with system site-packages

This adds a `-S`/`--use-system-packages` flag which passes the `--system-site-packages` flag to `virtualenv` allowing the package to re-use system packages, e.g. those installed frm upstream `python-*` packages, and still vendor unavailable packages inside it's virtualenv. :cake: 

I've found this hybrid approach is great for packaging up tools were you want an admin to still be able to control updates to upstream packages from `apt` rather than worrying about the tools own virtualenv, but I've previously had to do it by building the package with vendored deps (including all related build tools, like python-dev, cffi etc.), and then removing the "no-global-site-packages.txt" file in the venv, and removing any vendored package inside too.
This makes that easier by allowing the venv to share site-packages during the build process.

Of course the big advantage to virtualenvs is not using global site-packages, so the default for the flag is `False`. :grin: