conform isolated system to new early node choice

* Randomly chose an instance in the controller instance group for which
to control the isolated node run. Note the chosen instance via a job
controller_node field

awx/main/migrations/0040_v330_unifiedjob_controller_node.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-05-25 18:58
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0039_v330_custom_venv_help_text'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='unifiedjob',
+            name='controller_node',
+            field=models.TextField(blank=True, default=b'', editable=False, help_text='The instance that managed the isolated execution environment.'),
+        ),
+    ]