MAR-132: Fix virtual environment set-up in wsgi_website role:

- Install more up-to-date version of pip. This fixes some issues
related to pip freeze detecting argparse and wsgiref in virtualenv
as separate packages.
- Remove the pkg-resources package from virtualenv. Workaround for
Debian-specific behaviour. More details at:
- https://github.com/pypa/pip/issues/4022
- https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1635463
- https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=871790
- Deploy Gunicorn requirements file without switching to website admin
user (otherwise Ansible fails to template the file due to
permissions restrictions).
- Updated test for checking packages installed in the virtualenv.