Allow HdfsClient.mkdir(parents=False, raise=False)

It was just laziness from my part that I did not do this in previous
patches.
Use NotImplementedError in HdfsClient.mkdir()

It's more proper to raise NotImplementedError rather than raising an
AssertionError
Merge pull request #405 from Tarrasch/cleanup-hadoopcli-mkdir

Cleanup hadoopcli mkdir