Make InstanceLink target non-nullable

InstanceLink target should not be null.

Should be safe to set to null=False, because we have
a custom RunPython method to explicitly set
target to a proper key.

Also, add new test to test_migrations
which ensures data integrity after migrating
the receptor address model changes.

Signed-off-by: Seth Foster <fosterbseth@gmail.com>

awx/main/migrations/0189_inbound_hop_nodes.py

@@ -134,4 +134,11 @@ class Migration(migrations.Migration):
             name='peers',
             field=models.ManyToManyField(related_name='peers_from', through='main.InstanceLink', to='main.receptoraddress'),
         ),
+        migrations.AlterField(
+            model_name='instancelink',
+            name='target',
+            field=models.ForeignKey(
+                help_text='The target receptor address of this peer link.', on_delete=django.db.models.deletion.CASCADE, to='main.receptoraddress'
+            ),
+        ),
     ]