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trial_keeper use 50070 port to connect to webhdfs server, and PAI use a mapping method to map 50070 port to 5070 port to visit restful server, this method has some risk for PAI may not support this kind of mapping in later release.Now use Pylon path(/webhdfs/api/v1) instead of 50070 port in webhdfs client of trial_keeper, the path is transmitted in trainingService.
In this pr, we have these changes:

1. Change to use webhdfs path instead of 50070 port in hdfs client.
2. Change to use new hdfs package "PythonWebHDFS", which is build to support pylon by myself. You could test the new function from "sparksnail/nni:dev-pai" image to test pai trainingService.
3. Update some variables' name according to comments.