Django 1.8 fails with an ImproperlyConfigured exception when `fields` or
`exclude` isn't set:

```
class Meta:
 fields = '__all__'
 ```

on all descendents of ModelForm.

Omitting this is deprecated in 1.7:

https://docs.djangoproject.com/en/1.7/topics/forms/modelforms/#selecting-the-fields-to-use

Change-Id: I168161d352db7f0224323540dfa79a29dd28a47c
diff --git a/xos/admin.py b/xos/admin.py
index 0120e66..9ce1bc4 100644
--- a/xos/admin.py
+++ b/xos/admin.py
@@ -88,6 +88,7 @@
 
     class Meta:
         model = VTRTenant
+        fields = '__all__'
 
 class VTRTenantAdmin(ReadOnlyAwareAdmin):
     list_display = ('backend_status_icon', 'id', 'target', 'test', 'argument' )