Changed Product library to read data from os-release or libzypp

- In installation it is only libzypp
- On running system, first read from os-release, then libzypp
Test case adapted to the new behavior
Changes and version
Added function comments and little refactoring
Enhanced test readability
Merge pull request #229 from kobliha/Product_lazy_loading

Changed Product library to (lazy-)load information from os-release and libzypp (if needed)