Cache preferred instance groups

When creating unified job, stash the list of pk values from the
instance groups returned from preferred_instance_groups so that the
task management system does not need to call out to this method
repeatedly.

.preferred_instance_groups_cache is the new field

awx/main/models/jobs.py

@@ -1213,6 +1213,9 @@ class SystemJob(UnifiedJob, SystemJobOptions, JobNotificationMixin):
 
     extra_vars_dict = VarsDictProperty('extra_vars', True)
 
+    def _set_default_dependencies_processed(self):
+        self.dependencies_processed = True
+
     @classmethod
     def _get_parent_field_name(cls):
         return 'system_job_template'