nsd4slave backend

This backend sends nsd-control commands to a remote NSD4 server. It
depends on the multi backend and on another master backend, most likely
PowerDNS.

Change-Id: I722210de97b207d318f300092f4e3be08e894961
Implements: blueprint nsd4-slave-backend

designate/exceptions.py

@@ -34,6 +34,10 @@ class Backend(Exception):
     pass
 
 
+class NSD4SlaveBackendError(Backend):
+    pass
+
+
 class NotImplemented(Base, NotImplementedError):
     pass