Minor cleanup.

* Fix typos in README
* Wrap variables in quotes. Script failed if path to ISO contained spaces.
* Implement some suggestions from Google shell style guide.
* Address some shellcheck findings.
* Add a debug mode.
* Remove unused options.
* Move some repeated code to a function.

Thanks for uploading this project to github. Just got our first of many new servers this week and am going to give this a try over the weekend.