Add raise_if_exists flag to hdfs_client.mkdir()

It is *way* to complicated to actually implement it. So I just went with
assertions to make sure that the programmers who want can explicitly
state their intent.  See spotify/luigi#393 for a discussion about this.

Oh, and I also started to use it at one place to get the ball rolling.
Push down parent and raise_if_exists flags to abc

From real life experience of how hdfs.mkdir() behavior changed due to
lack of explicitness, we think it's good for all FileSystem
implementations to encourage explicitness in their implementations of
mkdir().

Discussion in spotify/luigi#393
Sync mkdir raise_if_exists=False up to Snakebite

This will *slightly* break backwards compatibility. But the behavior was
really undefined before so.