Fix race condition in SnakebiteHdfsClient.mkdir()

Rewritten in EAFP style:
https://docs.python.org/2/glossary.html#term-eafp
Security fix
Merge pull request #404 from Tarrasch/fix-snakebite-mkdir-race

Fix race condition in SnakebiteHdfsClient.mkdir()
Uses mkdir function in HdfsAtomicWritePipe init