BZ 1127297: remove hosts from hostgroup when deleting hostgroups

We were already doing this correctly on deployment delete, but
this commit moves it down one level, so any time we remove a
DeploymentRoleHostgroup, the hosts are removed too -- which
handles layout changes properly.
Merge pull request #260 from sseago/BZ1127297

BZ 1127297: remove hosts from hostgroup when deleting hostgroups