enforce 0 <= Instance.capacity_adjustment

see: https://github.com/ansible/tower/issues/2839

awx/main/migrations/0049_v330_validate_instance_capacity_adjustment.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.11 on 2018-08-17 16:13
+from __future__ import unicode_literals
+
+from decimal import Decimal
+import django.core.validators
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('main', '0048_v330_django_created_modified_by_model_name'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='instance',
+            name='capacity_adjustment',
+            field=models.DecimalField(decimal_places=2, default=Decimal('1'), max_digits=3, validators=[django.core.validators.MinValueValidator(0)]),
+        ),
+    ]

awx/main/models/ha.py

@@ -6,6 +6,7 @@ import random
 from decimal import Decimal
 
 from django.core.exceptions import ValidationError
+from django.core.validators import MinValueValidator
 from django.db import models, connection
 from django.db.models.signals import post_save, post_delete
 from django.dispatch import receiver
@@ -81,6 +82,7 @@ class Instance(HasPolicyEditsMixin, BaseModel):
         default=Decimal(1.0),
         max_digits=3,
         decimal_places=2,
+        validators=[MinValueValidator(0)]
     )
     enabled = models.BooleanField(
         default=True