Add support for extensions to prepend configuration for the db_driver option. If "cannotBeOverwritten()" is called then any extension trying to prepend the configuration can not modify the configuration value which makes the application configuration have to be more explicit. Having this support is useful where a child bundle to the FOSOAuthServerBundle might already know the DB driver that needs to be used.
Merge pull request #202 from Limelyte/allow-db-driver-override

Add support for extensions to prepend configuration for the db_driver option