correctly cascade set null

* It's problematic to delete an instance that is referenced by a foreign
key; where the referening model is one that has a Polymorphic parent.
* Specifically, when Django goes to nullify the relationship it relies
on the related instances[0] class type to issue a query to decide what
to nullify. So if the foreignkey references multiple different types
(i.e. ProjectUpdate, Job) then only 1 of those class types will get
nullified. The end result is an IntegrityError when delete() is called.
* This changeset ensures that the parent Polymorphic class is queried so
that all the foreignkey entries are nullified
* Also remove old Django "hack" that doesn't work with Django 1.11

awx/main/models/unified_jobs.py

@@ -38,6 +38,7 @@ from awx.main.utils import (
     copy_model_by_class, copy_m2m_relationships,
     get_type_for_model, parse_yaml_or_json
 )
+from awx.main.utils import polymorphic
 from awx.main.constants import ACTIVE_STATES, CAN_CANCEL
 from awx.main.redact import UriCleaner, REPLACE_STR
 from awx.main.consumers import emit_channel_notification
@@ -89,9 +90,6 @@ class UnifiedJobTemplate(PolymorphicModel, CommonModelNameNotUnique, Notificatio
 
     ALL_STATUS_CHOICES = OrderedDict(PROJECT_STATUS_CHOICES + INVENTORY_SOURCE_STATUS_CHOICES + JOB_TEMPLATE_STATUS_CHOICES + DEPRECATED_STATUS_CHOICES).items()
 
-    # NOTE: Working around a django-polymorphic issue: https://github.com/django-polymorphic/django-polymorphic/issues/229
-    base_manager_name = 'base_objects'
-
     class Meta:
         app_label = 'main'
         # unique_together here is intentionally commented out. Please make sure sub-classes of this model
@@ -536,9 +534,6 @@ class UnifiedJob(PolymorphicModel, PasswordFieldsModel, CommonModelNameNotUnique
 
     PASSWORD_FIELDS = ('start_args',)
 
-    # NOTE: Working around a django-polymorphic issue: https://github.com/django-polymorphic/django-polymorphic/issues/229
-    base_manager_name = 'base_objects'
-
     class Meta:
         app_label = 'main'
 
@@ -669,7 +664,7 @@ class UnifiedJob(PolymorphicModel, PasswordFieldsModel, CommonModelNameNotUnique
         blank=True,
         null=True,
         default=None,
-        on_delete=models.SET_NULL,
+        on_delete=polymorphic.SET_NULL,
         help_text=_('The Rampart/Instance group the job was run under'),
     )
     credentials = models.ManyToManyField(