fix celery task reaper

* celery workers have internal queue names that are named after the
system hostname. This may differ from what tower knows the host by,
Instance.hostname
This adds a mapping so we can convert internal celery names to Instance
names for purposes of reaping jobs.

awx/main/migrations/0041_v330_instance_system_hostname.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-06-14 17:23
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0040_v330_unifiedjob_controller_node'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='instance',
+            name='system_hostname',
+            field=models.CharField(default=b'', max_length=255, unique=True),
+        ),
+    ]