Add 'service' pdns to designate backend to fix reclass Application

Change-Id: I03f185d206aa1194f701f714ebf625b43ad24e42
diff --git a/designate/server/backend/pdns.yml b/designate/server/backend/pdns.yml
index 50ec3a2..45ad0b7 100644
--- a/designate/server/backend/pdns.yml
+++ b/designate/server/backend/pdns.yml
@@ -1,3 +1,5 @@
+classes:
+- service.powerdns.server.single
 parameters:
   designate:
     server:
@@ -7,7 +9,6 @@
           api_endpoint: ${_param:designate_pdns_api_endpoint}
   powerdns:
     server:
-      enabled: true
       bind:
         address: ${_param:single_address}
         port: 53