Very first attempt to implement systemd template.

Guys, this comes too erly for being really usefull. Still I believe it can be merged.

**However**, systemd test requires to change OVAL version for ssg/RHEL/7/ to 5.11. And also you will need to use --skip-valid during scanning. That is problemating because OVAL 5.11 was not yet released. I am not sure, what the release schedule is. :-( I am not sure if SSG wants to use OVAL 5.11 feature before it is released...

Therefore, I decided to contribute only template file and not the expansion (the `make copy`).