Add list_zones to AutoArpa

- This makes it possible to use dynamic zone config with AutoArpa
- Document what gotchas that entails
- Add tests both with another source (for generating zones to populate) and without (no zones to populate)

octodns/processor/arpa.py

@@ -72,3 +72,6 @@ class AutoArpa(BaseProcessor):
         self.log.info(
             'populate:   found %s records', len(zone.records) - before
         )
+
+    def list_zones(self):
+        return set()