[tune] Initial Commit for Tune CLI (#3983)

This introduces a light CLI for Tune.

python/ray/tune/tests/test_ray_trial_executor.py

@@ -18,6 +18,7 @@ class RayTrialExecutorTest(unittest.TestCase):
     def setUp(self):
         self.trial_executor = RayTrialExecutor(queue_trials=False)
         ray.init()
+        _register_all()  # Needed for flaky tests
 
     def tearDown(self):
         ray.shutdown()