Add support for environment variables.

We should add a note after the environment variables section that you can see what the CLI startup script uses for these values by setting the environment variable debug=true:

```
debug=true kite-dataset info users
```

That is how we recommend debugging any issues related to not finding the correct Hadoop installation. That will show all of the values of the above variables actually used, so the caller can inspect and possibly override them.

A quick note on environment variables. This uses the convention that things that should be persistent environment variables, like where Hadoop is installed (HADOOP_HOME) are in all caps, while things that you would add only for a single run, like debug, are in lower case.