Changed way how to determine THE PRODUCT bnc#871261

- In installation, update, auto-update, AutoYast, etc., /content file should be used. On the other hand, /etc/os-release should be used in running system, Live installation, etc. All in all, whether /content exists, it should be used, otherwise it's /etc/os-release
- This change makes easy to distinguish between the cases just by checking which file exists
- Additionally it throws an exception in neither of them exist
- Comes with extensive test cases!

Even if you love the change and give it LGTM, don't merge it ;) I'll add changes later.