Adding CLI support and tests for System Group.

Initial support and CLI CRUD tests for system groups. Sadly this cannot
yet safely be merged due to the fact that there are 2 major bugzilla
issues blocking assertions, deletions and updates.

* https://bugzilla.redhat.com/show_bug.cgi?id=1080475
* https://bugzilla.redhat.com/show_bug.cgi?id=1084240

This handles Issue #170.