Switch to modular file system for hdfs (tensorflow#1309)

* Switch to modular file system for hdfs

This PR is part of the effort to switch to modular file system for hdfs.
When TF_ENABLE_LEGACY_FILESYSTEM=1 is provided, old behavior will
be preserved.

Signed-off-by: Yong Tang <[email protected]>

* Build against tf-nightly

Signed-off-by: Yong Tang <[email protected]>

* Update tests

Signed-off-by: Yong Tang <[email protected]>

* Adjust the if else logic, follow review comment

Signed-off-by: Yong Tang <[email protected]>

Author: yongtang

tensorflow_io/core/plugins/file_system_plugins.cc

@@ -30,13 +30,12 @@ void TF_InitPlugin(TF_FilesystemPluginInfo* info) {
                                              sizeof(info->ops[0])));
   tensorflow::io::az::ProvideFilesystemSupportFor(&info->ops[0], "az");
   tensorflow::io::http::ProvideFilesystemSupportFor(&info->ops[1], "http");
+  tensorflow::io::s3::ProvideFilesystemSupportFor(&info->ops[2], "s3e");
   if (enable_legacy == "true" || enable_legacy == "1") {
-    tensorflow::io::s3::ProvideFilesystemSupportFor(&info->ops[2], "s3e");
     tensorflow::io::hdfs::ProvideFilesystemSupportFor(&info->ops[3], "hdfse");
     tensorflow::io::hdfs::ProvideFilesystemSupportFor(&info->ops[4], "viewfse");
     tensorflow::io::hdfs::ProvideFilesystemSupportFor(&info->ops[5], "hare");
   } else {
-    tensorflow::io::s3::ProvideFilesystemSupportFor(&info->ops[2], "s3");
     tensorflow::io::hdfs::ProvideFilesystemSupportFor(&info->ops[3], "hdfs");
     tensorflow::io::hdfs::ProvideFilesystemSupportFor(&info->ops[4], "viewfs");
     tensorflow::io::hdfs::ProvideFilesystemSupportFor(&info->ops[5], "har");