Distcp should add a default atomicWorkPath properties when using atomic

Distcp should add a default AtomicWorkPath properties when using atomic

Distcp#configureOutputFormat using code below to generate atomic work path,

    if (context.shouldAtomicCommit()) {
      Path workDir = context.getAtomicWorkPath();
      if (workDir == null) {
        workDir = targetPath.getParent();
      }
      workDir = new Path(workDir, WIP_PREFIX + targetPath.getName()
                                + rand.nextInt());

When atomic is set and AtomicWorkPath == null, distcp will get the parent of current WorkDir. In this case, if workdir is "/", the parent will be null, wich means
workDir = new Path(workDir, WIP_PREFIX + targetPath.getName() + rand.nextInt()); will throw a nullpoint exception.