[WIP] Add CUSTOM_CONFIG_TXT parameter

[denismosolov]

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.