Docker registry HDFS driver

This is a docker-registry backend driver for Hadoop's HDFS.

Dockerized version

There is a docker container with this

Hacking

This is a very quickly written first pass at HDFS support. It's mainly designed to be functional with little consideration for performance or reliability. So the todo list is a little long, sorry:

• Tests. Any tests would be nice.
• This driver uses hadoopy which fires off a java subprocess for every single request. Which is amazingly slow! It would be better to begin a migration to snakebite for reads. However it needs to support hadoop 2.4.x and eventually it would be nice if it supported writes...
• Caching! This uses the redis system for memory caching - stolen from the file driver. Is that the best way?
• Caching - again! This driver caches to the local fs. It doesn't manage this cache; it probably should.
• HDFS auth support.
• What if multiple registries are accessing the same HDFS store? Do we need locking? Is the docker-registry driver api expressive enough to note where locking is required? If we need locking, can we make that configurable/pluggable so that zookeeper, etcd, etc could be supported? And might locking more appropriately exist on a higher level? These questions and more can be answered if this TODO item gets done!