Define a Node class to manage Ray processes. (#3733)

* Implement Node class and move most of services.py into it.

* Wait for nodes as they are added to the cluster.

* Fix Redis authentication bug.

* Fix bug in client table ordering.

* Address comments.

* Kill raylet before plasma store in test.

* Minor

test/multi_node_test.py

@@ -181,7 +181,6 @@ print("success")
         out = run_string_as_driver(driver_script2)
         # Make sure the first driver ran to completion.
         assert "success" in out
-        assert ray.services.all_processes_alive()
 
 
 @pytest.fixture