Fix system jobs not starting due to task manager and cred changes

* Task Manager logic wasn't assigning default instance group on system
  jobs
* Task credential changes assumed the model would have a credential
* Fix up an innocuous error symlinking rdb.py if it already exists

awx/main/models/jobs.py

@@ -1344,6 +1344,10 @@ class SystemJob(UnifiedJob, SystemJobOptions, JobNotificationMixin):
     def task_impact(self):
         return 150
 
+    @property
+    def preferred_instance_groups(self):
+        return self.global_instance_groups
+
     '''
     JobNotificationMixin
     '''

awx/main/tasks.py

@@ -737,8 +737,10 @@ class BaseTask(Task):
             # handle custom injectors specified on the CredentialType
             if hasattr(instance, 'all_credentials'):
                 credentials = instance.all_credentials
-            else:
+            elif hasattr(instance, 'credential'):
                 credentials = [instance.credential]
+            else:
+                credentials = []
             for credential in credentials:
                 if credential:
                     credential.credential_type.inject_credential(

tools/docker-compose/start_development.sh

@@ -25,7 +25,7 @@ fi
 
 cp -nR /tmp/ansible_tower.egg-info /tower_devel/ || true
 cp /tmp/ansible-tower.egg-link /venv/tower/lib/python2.7/site-packages/ansible-tower.egg-link
-ln -s /tower_devel/tools/rdb.py /venv/tower/lib/python2.7/site-packages/rdb.py
+ln -s /tower_devel/tools/rdb.py /venv/tower/lib/python2.7/site-packages/rdb.py || true
 yes | cp -rf /tower_devel/tools/docker-compose/supervisor.conf /supervisor.conf
 
 # Tower bootstrapping