allow to specify directory from which to run the "pip install" command

If a requirements.txt includes relative paths (like `../src`), it seems like `pip install` must be run from within that directory. This works correctly for the first installation when the virtualenv is created, but if a change is detected in requirements.txt by puppet, `pip install` will be executed again, but fail.

This change fixes this for me.

As `$cwd` was already defined, I renamed that to `$rootdir` as that seems to be a more apropriate name.