support workflow convergence nodes

* remove convergence restriction in API
* change task manager logic to be aware of and support convergence nodes

awx/main/migrations/0050_v331_workflow_convergence.py

@@ -0,0 +1,20 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-09-28 14:23
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0049_v330_validate_instance_capacity_adjustment'),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name='workflowjobnode',
+            name='do_not_run',
+            field=models.BooleanField(default=False),
+        ),
+    ]