Add configuration keys for retry policy in WebHDFSFileSystem

Currently the WebHDFS client reuses the configuration of DFSClient to construct its RetryPolicy. This is problematic because:

1. It makes the WebHDFS client depends on the DFSClient.
2. The default values of the RetryPolicy of DFSClient might be ill fit for the WebHDFS client, which transfers all data using HTTP.

This JIRA proposes to introduce new configuration keys for WebHDFS to resolve this problem.