Handle constants from .dist

I have a use case where the mcrypt cipher and mode are stored into the .dist file.

```
parameters:
    cipher: !php/const:MCRYPT_RIJNDAEL_128
    mode: !php/const:MCRYPT_MODE_CBC
```

After the build of the parameters.yml file, these two keys are set to null.

I can submit a PR to correct that case but I just wanted to know your opinion about this issue.

Regards,