Fixes #5094: Create ncf-api-virtualenv

Ticket: http://www.rudder-project.org/redmine/issues/5094

To be merged in 2.11
This file is the list of requirements for the ncf-api application. It should most definitely live with the code, in the ncf repo. Having it here just means that it will never get updated when a developer adds a dependency, and that it will not be available to people who want to set up the API without using this "special" package.