[FLINK-1266] Generalize DistributedFileSystem implementation
to HadoopFileSystem wrapper, which supports all subclasses of org.apache.hadoop.fs.FileSystem.
This allows us to let users use all file systems with support for HDFS.
The change has been tested with Tachyon, Google Cloud Storage Hadoop Adapter and HDFS.

The change also cleans up the Hadoop dependency exclusions.