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ARROW-2661: [Python] Adding the ability to programmatically pass hdfs configration key/value pairs via pyarrow

https://issues.apache.org/jira/browse/ARROW-2661

Both the JNI and libhdfs3 support hdfsBuilderConfSetStr so we can utilize that to allow passing arbitrary configuration values for hdfs connection similiar to how https://hdfs3.readthedocs.io/en/latest/hdfs.html supports passing them.

I've added a param called `extra_conf` to facilitate it in pyarrow, such as:

```python
import pyarrow
conf = {"dfs.nameservices": "nameservice1",
        "dfs.ha.namenodes.nameservice1": "namenode113,namenode188",
        "dfs.namenode.rpc-address.nameservice1.namenode113": "hostname_of_server1:8020",
        "dfs.namenode.rpc-address.nameservice1.namenode188": "hostname_of_server2:8020",
        "dfs.namenode.http-address.nameservice1.namenode188": "hostname_of_server1:50070",
        "dfs.namenode.http-address.nameservice1.namenode188": "hostname_of_server2:50070",
        "hadoop.security.authentication": "kerberos"
}
hdfs = pyarrow.hdfs.connect(host='nameservice1', driver='libhdfs3', extra_conf=conf)
```

Author: Matthew Topol <mtopol@factset.com>

Closes #2097 from zeroshade/configs and squashes the following commits:

047dd4b1 <Matthew Topol> forgot to use make format to fix the order of includes. oops
d27e3c3e <Matthew Topol> switching to unordered_map
858b44bb <Matthew Topol> missed a flake8 spot
77eeae09 <Matthew Topol>  Adding the ability to programmatically pass hdfs configuration key/value pairs in the C++ and via pyarrow