Defaulted hdfs to using hostname and port from env

If not specified in the URI we will default to using the hostname as defined in `PYFS_HADOOP_NAMENODE_ADDR`. If not specified in the URI we will default the port number to `PYFS_HADOOP_NAMENODE_PORT` or 50070.

The following URIs can now be opened, where the env defines the host:

* `hdfs://`
* `hdfs:///`
* `hdfs:///subfolder`
* `hdfs://:80`
* `hdfs://:80/`
* `hdfs://host:`
* `hdfs://host:/`
* `hdfs://host:/subfolder`
* `hdfs://host:80`
* `hdfs://host:80/`
* `hdfs://host:80/subfolder`

Naice.