Actually poll for zone deletes

- This implements a few `TODO`s in the code around actually polling
nameservers for zone deletes. I found this necessary for changes
that were truing up missed DELETEs, and wanted to make a poll/
update_status call to verify deletion across a fleet of nameservers
when one was down.
- This originally failed in PowerDNS, because in older versions
that are still in all the distro repos, PDNS responded with a
NOERROR and empty answer section when it got a query for a zone
it knew nothing about. NEAT.
http://blog.powerdns.com/2015/03/02/from-noerror-to-refused/

- NOTE: This increases timeout for functional tests and we maybe
shouldn't do that.

Change-Id: Ied1e5daf4798127e00a06265164759d98fc9ba72

designate/tests/unit/test_mdns/test_notify.py

@@ -161,6 +161,7 @@ class MdnsNotifyTest(base.BaseTestCase):
             rcode=Mock(return_value=dns.rcode.NOERROR),
             # rcode is NOERROR but (flags & dns.flags.AA) gives 0
             flags=0,
+            answer=['answer'],
         )
         self.notify._send_dns_message = Mock(return_value=response)
 
@@ -176,7 +177,8 @@ class MdnsNotifyTest(base.BaseTestCase):
             rcode=Mock(return_value=dns.rcode.NOERROR),
             # rcode is NOERROR but flags are not NOERROR
             flags=123,
-            ednsflags=321
+            ednsflags=321,
+            answer=['answer'],
         )
         self.notify._send_dns_message = Mock(return_value=response)