Extend default config instead overwrite

[likemusic]

Description

Default FormElementDependenceController config should be extended by custom config but not overridden.

Manual testing scenarios

1. Add custom options to FormElementDependenceController config;
2. Open System config page.

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)

[VladimirZaets]

Hi, @likemusic, I took your PR into processing, thank you for collaboration.

[magento-engcom-team]

Hi @likemusic. Thank you for your contribution.
Changes from your Pull Request will be available with the upcoming 2.2.6 release.

Please, consider to port this solution to 2.3 release line.
You may use Porting tool to port commits automatically.