Add a total maximum time that a machine is allowed to stick around before we terminate it more forcefully.

test/test_buildman.py

@@ -159,8 +159,13 @@ class TestEphemeral(unittest.TestCase):
 
   @async_test
   def test_heartbeat_response(self):
+    expiration_timestamp = time.time() + 60
     builder_result = Mock(spec=etcd.EtcdResult)
-    builder_result.value = {'builder_id': '123', 'expiration': '123'}
+    builder_result.value = {
+        'builder_id': '123',
+        'expiration': expiration_timestamp,
+        'max_expiration': expiration_timestamp,
+    }
     self.etcd_client_mock.read = Mock(return_value=builder_result)
 
     yield From(self.manager.job_heartbeat(self.mock_job))