Fix deprecation warning when importing luigi.hdfs.

You will still see a deprecation warning when creating an
HdfsTarget if you haven't specified a config.

example:

```
>>> import luigi.hdfs
>>> luigi.hdfs.HdfsTarget()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "luigi/hdfs.py", line 666, in __init__
    assert is_tmp
AssertionError
>>> luigi.hdfs.HdfsTarget(path='/tmp')
luigi/hdfs.py:548: UserWarning: Deprecated: Specify 'client: hadoopcli' in config
  warnings.warn("Deprecated: Specify 'client: hadoopcli' in config")
<luigi.hdfs.HdfsTarget object at 0x108855e10>
```

Refs #426