Included recipes for Datanode and Namenode

Signed-off-by: Shashank Sahni <shredder12@gmail.com>
Added mapred.system.dir config option

* Created a new attribute, [hadoop][mapred_system_dir]
* Updated mapred-site.xml.erb
* Added directory creation block in prepare.rb recipe
* Updated README to reflect the changes

Signed-off-by: Shashank Sahni <shredder12@gmail.com>
Added dfs.permissions.supergroup in hdfs-site.xml

This option is set to "hadoop" group. This option sets the
superusergroup of hdfs. If not set to "hadoop", mapred user doesn't
have the permission to create a directory.

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