move auto-arpa to prepend post_processors

Preferable to have it run later after other processors have had their
change to add/remove records. Otherwise there may be PTRs created for
things that processors have filtered out. It's always possible to
manually include it in the appropriate places if you need finger grained
control.

octodns/manager.py

@@ -127,13 +127,15 @@ class Manager(object):
 
         if self.auto_arpa:
             self.log.info(
-                '__init__: adding auto-arpa to processors and providers, appending it to global_processors list'
+                '__init__: adding auto-arpa to processors and providers, prepending it to global_post_processors list'
             )
             kwargs = self.auto_arpa if isinstance(auto_arpa, dict) else {}
             auto_arpa = AutoArpa('auto-arpa', **kwargs)
             self.providers[auto_arpa.name] = auto_arpa
             self.processors[auto_arpa.name] = auto_arpa
-            self.global_processors.append(auto_arpa.name)
+            self.global_post_processors = [
+                auto_arpa.name
+            ] + self.global_post_processors
 
         plan_outputs_config = manager_config.get(
             'plan_outputs',