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'.