Use worker harness container corresponding to SDK version

----Release Notes----
[]
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=122697631

Author: charlesccychen

google/cloud/dataflow/internal/apiclient.py

@@ -239,6 +239,11 @@ def __init__(self, packages, options, environment_version):
     if self.worker_options.worker_harness_container_image:
       pool.workerHarnessContainerImage = (
           self.worker_options.worker_harness_container_image)
+    else:
+      # Default to using the worker harness container image for the current SDK
+      # version.
+      pool.workerHarnessContainerImage = (
+          'dataflow.gcr.io/v1beta3/python:%s' % version.__version__)
     if self.worker_options.teardown_policy:
       if self.worker_options.teardown_policy == 'TEARDOWN_NEVER':
         pool.teardownPolicy = (

google/cloud/dataflow/utils/options.py

@@ -315,7 +315,10 @@ def _add_argparse_args(cls, parser):
     parser.add_argument(
         '--worker_harness_container_image',
         default=None,
-        help='Docker image to use for the worker harness.')
+        help=('Docker registry location of container image to use for the '
+              'worker harness. Default is the container for the version of the '
+              'SDK. Note: currently, only approved Google Cloud Dataflow '
+              'container images may be used here.'))
     parser.add_argument(
         '--teardown_policy',
         choices=['TEARDOWN_ALWAYS', 'TEARDOWN_NEVER', 'TEARDOWN_ON_SUCCESS'],