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.
Removed __init__ as it not needed anymore.
Merge pull request #633 from omaciel/system-groups-cli

Adding CLI support and tests for System Group.