Allow multiple --site options in commands

Hi,

I've added the availability to give multiple website identifiers using SonataPageBundle commands.

This is not a BC break as it continues to work as today.

Currently: following commands are available
-----------------------------------------------------------

**For a specific website:**
```bash
$ php app/console sonata:page:update-core-routes --site=1
$ php app/console sonata:page:create-snapshots --site=1
$ php app/console sonata:page:cleanup-snapshots --site=1
```

**For all websites:**
```bash
$ php app/console sonata:page:update-core-routes --site=all
$ php app/console sonata:page:create-snapshots --site=all
$ php app/console sonata:page:cleanup-snapshots --site=all
```

Added: allows multiple websites identifiers
-----------------------------------------------------------

```bash
$ php app/console sonata:page:update-core-routes --site=1 --site=2 --site=...
$ php app/console sonata:page:create-snapshots --site=1 --site=2 --site=...
$ php app/console sonata:page:cleanup-snapshots --site=1 --site=2 --site=...
```