Bug #2219 User <-> Hostgroup ACLs to follow the hostgroup hierarchy

In short, this pull requests ensures that given a user owns a top-level hostgroup, it will own its hostgroups subtree. 

Given a hostgroup that has lots of subhostgroups, it can be time consuming for the foreman admin to assign rights to people who own several hostgroups on a hierarchy by selecting each of the hostgroups. An example is provided in the issue tracker bug. 

http://projects.theforeman.org/issues/2219