Refactor backdrop app to generic gunicorn app

- Move pip download cache directory creation into it's own class in pp
  module
- Move generic gunicorn app creation into it's own defined type in pp
  module
- Modify hiera yaml to reflect refactor

This refactor allows us to set up stagecraft with the same mechanism
that backdrop is currently using. There is nothing backdrop specific in
the app setup.
Add stagecraft app
Add option to make a vhost only visible to us

This depends on configuration being set in pp-deployment
Add vcl and vhost for stagecraft

Stagecraft MVP is going to be locked down to just Aviation House and
Performance Platform, however, to take advantage of the load balancer
and make it easy to make public we're serving it out through the
frontend boxes.
fix some lint
Merge pull request #253 from alphagov/setup-stagecraft

Setup stagecraft