Fix designate bind backend

Change-Id: I7688059f1d48f4a5d1a729bd0857c0e39b2174b5
diff --git a/designate/server/backend/bind.yml b/designate/server/backend/bind.yml
index c057eb7..5836f24 100644
--- a/designate/server/backend/bind.yml
+++ b/designate/server/backend/bind.yml
@@ -1,3 +1,5 @@
+classes:
+- service.bind.server.single
 parameters:
   _param:
     designate_bind9_rndc_algorithm: hmac-sha512