polish get started tutorials

[QiJune]

• [x] Revise and polish elasticdl local tutorial, set up a mnist training experiment with minikube quickly #1912
• [ ] Add a deployment tutorial, save model when training, and deploy it with tf-serving
• [ ] Polish elasticdl cloud tutorial, set up a mnist training experiment with different environment: Alibaba Cloud/GKE/AWS/
• [x] Add a dataset preparation tutorial, show how to read data from different sources: RecordIO and MaxCompute Table
• [x] Add a tutorial on elastic scheduling of deep learning training jobs in public cloud environment