[ConsoleCommand] Added path of generated thumbnails to sonata:media:sync-thumbnails

Added path of generated thumbnails to ```sonata:media:sync-thumbnails``` command's output when verbosity level is at least 2 (executed with ```-v``` option).

Before:
```
$ app/console sonata:media:sync-thumbnails sonata.media.provider.youtube video -v
Loaded 300 medias for generating thumbs (provider: sonata.media.provider.youtube, context: video)
Generating thumbs for My awesome video - 150
Generating thumbs for Another video - 152
...
```

After:
```
$ app/console sonata:media:sync-thumbnails sonata.media.provider.youtube video -v
Loaded 300 medias for generating thumbs (provider: sonata.media.provider.youtube, context: video)
Generating thumbs for My awesome video - 150
[video_preview] => video/0001/01/thumb_150_video_preview.jpg
[video_small] => video/0001/01/thumb_150_video_small.jpg
[video_large] => video/0001/01/thumb_150_video_large.jpg
[admin] => video/0001/01/thumb_150_admin.jpg

Generating thumbs for Another video - 152
[video_preview] => video/0001/01/thumb_152_video_preview.jpg
[video_small] => video/0001/01/thumb_152_video_small.jpg
[video_large] => video/0001/01/thumb_152_video_large.jpg
[admin] => video/0001/01/thumb_152_admin.jpg
...
```

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT