plasma_datanode_init -blocksize <blocksize> <directory> <blocks>
Creates two files in directory: config and data. The data
file is created as empty sequence of blocks blocks, where every
block consists of blocksize bytes.
Also, a datanode identity is created (32 hex digits).
The identity and the blocksize are put into config.
After the directory is prepared with this command, it can be referenced
in the directory parameter of a datanode config file.
