Add missing related_name for IPSubnet

coin/resources/models.py

@@ -17,4 +17,5 @@ class IPSubnet(models.Model):
     inet = CidrAddressField()
     objects = NetManager()
     ip_pool = models.ForeignKey(IPPool)
-    offer_subscription = models.ForeignKey('offers.OfferSubscription')
+    offer_subscription = models.ForeignKey('offers.OfferSubscription',
+                                           related_name='ip_subnet')