Making the 3.0 directory structure question not asked unless you have a constant

Hi guys!

This follows symfony/symfony-standard#674

Basically, I'm seeing beginners choose "Y" for the Symfony 3 directory structure and then being very confused when their project doesn't look right. I realize this may be a bit controversial, but I just don't see the advantage yet of allowing people to install the "Symfony 3" directory structure until we're actually ready to release Symfony 3 and can update all the documentation accordingly. I've still made it possible to use the 3.0 dir structure, but only for someone who knows what they're doing (i.e. will set a constant).

I know I wasn't active in the conversation when this decision was made - so apologies for coming late - I just missed it before :).

Thanks!