Added another hostgroup association to deployment.

As it turns out, a Deployment not only has_many HostGroups (one per role
 -- i.e. controller, comute, etc), but there also needs to be a Hostgroup
at the deployment level.

So Deployment has two associations now:
1) belongs_to :hostgroup (this is the "deployment-level" Hostgroup
2) has_many :role_hostgroups (this is the "role-level" hostgroup

the role-level hostgroups are children of the deployment-level
hostgroup (which is in turn a child of the base hostgroup)