[Forwardport] magento/magento2#:11155 Refactor ConfigGenerator to replace/set crypt key instead of append

[nmalevanec]

Original Pull Request

#11155 by @renttek)

General

Fix the command "setup:config:set --key=..." to replace existing crypt key instead of append it.

Description

Originally a call so "bin/magento setup:config:set --key='foobar'" did
not replace the crypt key, but append it with a newline.

This does not only "break" handling of values, encryptwed with the old
key, but also don't allow use of new key, because the new key is
composed of the old one, a newline and the new one.

Fixed Issues (if relevant)

1. setup:config:set --key append instead of replace #11089: setup:config:set --key append instead of replace

Manual testing scenarios

1. Install Magento & get sure, that there is a crypt key defined in env.php
2. Call "bin/magento setup:config:set --key="
3. New crypt key in env.php should be your new key, instead of "\n"

Contribution checklist

• Pull request has a meaningful description of its purpose
• All commits are accompanied by meaningful commit messages
• All new or changed code is covered with unit/integration tests (if applicable)
• All automated tests passed successfully (all builds on Travis CI are green)