Don't use 'updated_at' to check service's status

Commit b9bae02af2 introduced
'last_seen_up' to check service in Liberty. Nova use 'updated_at'
to check that before Liberty. So we can remove legacy item now.
Note: 'last_seen_up' may be null at the beginning. so we still
need 'created_at' in this case.

Change-Id: I5a273c294341027fba66a54a540182fc890522db

nova/servicegroup/drivers/db.py

@@ -57,10 +57,8 @@ class DbDriver(base.Driver):
         """Moved from nova.utils
         Check whether a service is up based on last heartbeat.
         """
-        # Keep checking 'updated_at' if 'last_seen_up' isn't set.
-        # Should be able to use only 'last_seen_up' in the M release
         last_heartbeat = (service_ref.get('last_seen_up') or
-            service_ref['updated_at'] or service_ref['created_at'])
+            service_ref['created_at'])
         if isinstance(last_heartbeat, six.string_types):
             # NOTE(russellb) If this service_ref came in over rpc via
             # conductor, then the timestamp will be a string and needs to be

nova/tests/unit/servicegroup/test_db_servicegroup.py

@@ -64,6 +64,13 @@ class DBServiceGroupTestCase(test.NoDBTestCase):
         result = self.servicegroup_api.service_is_up(service)
         self.assertFalse(result)
 
+        # "last_seen_up" is none before compute node reports its status,
+        # just use 'created_at' as last_heartbeat.
+        service.last_seen_up = None
+        service.created_at = timeutils.utcnow()
+        result = self.servicegroup_api.service_is_up(service)
+        self.assertTrue(result)
+
     def test_join(self):
         service = mock.MagicMock(report_interval=1)