Update functional.suite.dist.yml to handle a custom backend name

[scribam]

Description

In the file functional.suite.dist.yml, the value for the backend_name configuration is hardcoded. We should be able to customize the value by using the variable MAGENTO_BACKEND_NAME defined in the .env file.

magento2/dev/tests/acceptance/.env.example

Line 34 in b0babda

MAGENTO_BACKEND_NAME=

Manual testing scenarios

1. Setup Magento with a backend name different from admin
2. Setup acceptance tests with MAGENTO_BACKEND_NAME=<your_custom_backend_name> in your .env file
3. Run acceptance tests

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)

[magento-cicd2]

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