arrow

ARROW-557: [Python] Add option to explicitly opt in to HDFS tests, do not implicitly skip

I have

```
$ py.test pyarrow/tests/test_hdfs.py
================================== test session starts ==================================
platform linux2 -- Python 2.7.11, pytest-2.9.0, py-1.4.31, pluggy-0.3.1
rootdir: /home/wesm/code/arrow/python, inifile:
collected 15 items

pyarrow/tests/test_hdfs.py sssssssssssssss
```

But

```
$ py.test pyarrow/tests/test_hdfs.py --hdfs -v
================================== test session starts ==================================
platform linux2 -- Python 2.7.11, pytest-2.9.0, py-1.4.31, pluggy-0.3.1 -- /home/wesm/anaconda3/envs/py27/bin/python
cachedir: .cache
rootdir: /home/wesm/code/arrow/python, inifile:
collected 15 items

pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_close PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_download_upload PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_file_context_manager PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_ls PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_mkdir PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_orphaned_file PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_read_multiple_parquet_files SKIPPED
pyarrow/tests/test_hdfs.py::TestLibHdfs::test_hdfs_read_whole_file PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_close PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_download_upload PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_file_context_manager PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_ls PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_mkdir PASSED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_read_multiple_parquet_files SKIPPED
pyarrow/tests/test_hdfs.py::TestLibHdfs3::test_hdfs_read_whole_file PASSED
```

The `py.test pyarrow --only-hdfs` option will run only the HDFS tests.

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #353 from wesm/ARROW-557 and squashes the following commits:

52e03db [Wes McKinney] Add conftest.py file, hdfs group to opt in to HDFS tests with --hdfs