MAR-122: Fixed handling of complex package versions for pip with Ansible 2.x:

- Use list for installing specific version of pip in the "pipreqcheck" virtual
environment in common role.
- Removed superficious (and incorrect) definition for application admin username
in wsgi_website role.
- Updated test playbook for wsgi_website role to test the fix.
- Switched to passing-in package list for installing packages in virtual
environment instead of using "with_items" in wsgi_website role.