another approach to use spot instance for persistent training in AWS

[xiuliren]

• create a spot fleet or persistent request, it is a kind of persistent spot instance managed by AWS. define the training command by user_data of the spot instance.
• read all the data and configuration files in S3, save network to S3. This can be implemented using boto3 and remove the complex dependency of starcluster.
• whenever there is an update of saved network file, plot the learning curve online using Plotly.

[xiuliren]

created a pull request to implement this feature:
#94
need some more tests.