[DX] Normalize generation tools

Add an entity argument on the `doctrine:generate:crud` command

```
php app/console doctrine:generate:crud AcmeDemoBundle:Post
```

is equivalent to

```
php app/console doctrine:generate:crud --entity=AcmeDemoBundle:Post
```

Related to the issue #280
``$input->hasArgument('entity')`` is useless. We know that the input definition has this argument, as we defined it in the command