System Groups changed to Host Collections.

Updated CLI modules for system-group to now use host-collections command
and arguments. Also, cleaned up tests and unblocked suite since only the
'update' subcommand is yet to be implemented.

The CLI changes can be seen here:
https://github.com/Katello/hammer-cli-katello/pull/158 (already merged).

Sadly, since I don't have a system with these changes I was unable to
run tests.
Merge pull request #814 from omaciel/fix-system-groups

System Groups changed to Host Collections.