MAR-30: Implemented tests for role 'preseed':

- Added global flake8 configuration to allow up to 160 characters per line (for
Python linting checks).
- Updated gitignore file to ignore Molecule artefacts.
- Role is tested against 64-bit Debian Jessie and 64-bit Debian Stretch.
- Tests cover variations on optional parameter inclusion, as well as generated
preseed files.