[Proposal] Move the environment configuration to a dedicated file environments.php

Extracting the environment configuration in a separate file ```bootstrap/environments.php``` should improve maintainability :
- it allows team members to have their own environments and machine names, while keeping intact ```bootstrap/start.php``` in the project repository ;
- it eases future upgrade processes, as we have a good chance that ```bootstrap/start.php``` was not modified.

Any feedback welcome !