Added checks for Namenode format

* Introduced a new attribute, ['hadoop']['dfs_dir'], specifying the
parent directory of namenode/datanode directory.
* Updated recipe to create dfs_dir and apply appropriate permissions
* Updated namenode format resource to not run if NN directory exists.
This directory is created during format. Its existence means NN is
already formatted.

Signed-off-by: Shashank Sahni <shredder12@gmail.com>