Bug #2231: Hostgroup deletion follows adopt policy

Given a hostgroup hierarchy like:

/base
/base/test01
/base/test01/test02
/base/test01/test03

Currently, if /base/test01 is deleted, it leads to a clunky configuration

/base
/test02
/test03

This change makes a hostgroup adopt orphans so that removing test01 would lead to:

/base
/base/test02
/base/test03

http://projects.theforeman.org/issues/2231