MAR-30: Fixes for multiple platforms and more extensive testing:

- Include instructions on how to run tests against different platforms (Debian
versions).
- Use correct base box for Debian Stretch when testing role 'preseed'.
- Use fixed hostnames when runnig teets for role 'preseed'.
- Make sure to test preseed configuration file when providing overrides for a
host that does not include overrides as well in role 'preseed'.

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.