fixed isolated instance query

* Was considering an isolated instance: any instance that has at least 1
group with no controller. This is technically correct since an iso node
can not be a part of a non-iso group.
* The query is now more robust and considers a node an iso node if ALL
groups that a node belong to ALL have a controller.
* Also added better debugging for the special tower instance group
* Added a check for the existance of the special tower group so that
logs are less "messy" during the install process.

awx/main/models/ha.py

@@ -163,10 +163,6 @@ class InstanceGroup(models.Model, RelatedJobsMixin):
     def _get_related_jobs(self):
         return UnifiedJob.objects.filter(instance_group=self)
 
-    def add_all_non_iso_instances(self):
-        self.instances = Instance.objects.all_non_isolated()
-        self.save()
-
 
     class Meta:
         app_label = 'main'