Little fixes in etcd3 and kubernetes (#2689)

- Always pass etcd3 key revision as a string
- Make sure the leader key isn't unconditionally overwritten. It may happen that the leader heart-beat loop didn't run properly and the session has expired. In this case the leader may create a new session and a new leader key. But, there are chances that the other node already created a leader key and we don't want to overwrite it.
- Try to sync HA loops between nodes by adding 0.5 seconds to timeout on non-leader nodes

tests/test_kubernetes.py

@@ -315,7 +315,7 @@ class TestKubernetesConfigMaps(BaseTestKubernetes):
     def test_watch(self):
         self.k.set_ttl(10)
         self.k.watch(None, 0)
-        self.k.watch(None, 0)
+        self.k.watch('5', 0)
 
     def test_set_history_value(self):
         self.k.set_history_value('{}')