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: I639691b2d5471c384be8fe994725fc2a2d960012
diff --git a/xos/admin.py b/xos/admin.py
index 4bd99b6..27d97f9 100644
--- a/xos/admin.py
+++ b/xos/admin.py
@@ -94,6 +94,7 @@
 
     class Meta:
         model = VRouterTenant
+        fields = '__all__'
 
 class VRouterTenantAdmin(ReadOnlyAwareAdmin):
     list_display = ('backend_status_icon', 'id', 'subscriber_tenant', 'public_ip' )