Overhaul the hdfs client configuration

So the old syntax was something very hackish and very special-case oriented.

    [hdfs]
    use_snakebite: true

Now it becomes:

    [hdfs]
    client: snakebite

This patch also adds the configuration:

    [hdfs]
    client: snakebite_with_hadoopcli_fallback

This patch maintains backwards compatibility with old configuration.

This patch depend on spotify/luigi#395, so that must be merged before
this one.