Default firewall option

Lot of Symfony projet use only one main firewall, so we have to regularly to make this:

```php
$this->loginAs($this->getReference('user_support'), 'main');
```

The `main` keyword is very redundant. It would be great to add a `default_login_firewall` parameter to the bundle in order to just do this:

```php
$this->loginAs($this->getReference('user_support'));
```

Start by locating the bundle's loginAs helper and its configuration handling. Add support for a default_login_firewall parameter so the firewall argument can be omitted when using the default, while keeping explicit firewall arguments working; verify the behavior with the bundle's existing functional test suite.

Written by the indexing model from the issue text.