Closes #239

Please make sure all below checkboxes in the Mandatory section have been checked before submitting your PR (also don't forget to pay attention to the Depending on Circumstances section)

Mandatory

• The commit message is an imperative and starts with a capital letter (for example, Add something, Remove something, Fix something, etc.).
• I made sure I hadn't put a period at the end of the the commit message(s).

Depending on Circumstances

Some of the items in the section become mandatory if you are a first-time contributor and/or your changes are relevant to the items.

• Adding a new parameter I didn't forget to reflect it in README.md.
• Adding a new parameter or modifying an existing one I didn't break the alphabetical order.
• Adding a new utility written in Python I didn't forget to reflect it in pieman/README.md.
• Adding a new utility written in Python I didn't forget to increase the version number of the pieman package in pieman/setup.py, pieman/pieman/__init__.py and essentials.sh.
• (for first-time contributors) One of the commits in the pull request adds me to the Acknowledgements section in AUTHORS.md.