Description of problem:
Any user can add any system to their group, even if they don't own the system.

Version-Release number of selected component (if applicable):
unreleased

Steps to Reproduce:
1. Create a new group
2. From the group edit page, add a system which you do not own

Actual results:
Permitted to add the system.

Expected results:
Should only be allowed to add systems which you own. Otherwise it is a loophole through which users can grant themselves extra access to locked-down systems.