MAR-131: Added support for specifying Python version in wsgi_website role:

- Introduced additional role parameter for specifying the Python
version.
- Updated tests to verify new functionality.
- Fixed existing tests to account for differences between Python 2 and
Python 3 - including changes to WSGI test applications.
- Updated documentation, documenting new parameter and fixing one
minor typo.
- Updated release notes.
- Bumped default version of Gunicorn/futures used.

MAR-128: Upgraded tests for ROLE_NAME role:

- Switch to new Molecule configuration.
- Updated set-up playbook to use become: yes.
- Moved some preparatory steps outside of the main playbook (eases
idempotence tests).
- Updated tests to reference the yml inventory file.
- Updated tests to use new fixture (host instead of individual ones).
- Switched to extracting hostname instead of hard-coding it in a
couple of tests.
- Fixed some linting issues.
- Updated one of the tests that depend on output of pip freeze due to
new versions of packages coming out.
- Updated hostname to include Debian version (for future expansion).
- Renamded some test data to match new hostname.