[v1alpha2] Estimator support - Do not include evaluator in cluster spec

[xychu]

When using Estimator and tf.estimator.train_and_evaluate for distributed task,
the evaluator should not be included in the cluster spec.

Example of evaluator node (evaluator is not part of training cluster):

  cluster = {'chief': ['host0:2222'],
             'ps': ['host1:2222', 'host2:2222'],
             'worker': ['host3:2222', 'host4:2222', 'host5:2222']}
  os.environ['TF_CONFIG'] = json.dumps(
      {'cluster': cluster,
       'task': {'type': 'evaluator', 'index': 0}})
  config = RunConfig()
  assert config.master == ''
  assert config.evaluator_master == ''
  assert config.task_id == 0
  assert config.num_ps_replicas == 0
  assert config.num_worker_replicas == 0
  assert config.cluster_spec == {}
  assert config.task_type == 'evaluator'
  assert not config.is_chief

Refs:
https://www.tensorflow.org/api_docs/python/tf/estimator/RunConfig
https://www.tensorflow.org/api_docs/python/tf/estimator/train_and_evaluate

---

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[gaocegege]

Thanks for your contribution!

/lgtm
/approve

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[coveralls]

Coverage decreased (-0.05%) to 54.339% when pulling 7d1d084 on xychu:estimator_support into ee1709f on kubeflow:master.