make clean_dirs configurable

[Stevie-Ray]

Hi Matthias,

I was wondering, since Magento changed it's way of using var/generation I'd like to suggest to use:

        'clean_dirs' => explode(',',getenv('CLEAN_DIRS')) ?: [
            'var/cache',
            'var/di',
            'var/generation',
        ],

So we can use: CLEAN_DIRS = 'var/cache,generated/code,generated/metadata'

Let me know what you think,

Stevie