Fixes #5094: Create ncf-api-virtualenv

Ticket: http://www.rudder-project.org/redmine/issues/5094

To be merged in 2.11
Do we really want to install these 2 files? As I mentoined above, I think requirements.txt should be bundled with the API, so it will already be on the target hosts. As for virtualenv.py, I really see no reason for us to provide it. I see it as a build tool. If the user wants a virtualenv.py script, he can install one (which will likely be more up to date than ours). Also, if we install this, we need to maintain it, and the less we have to maintain, the better!