[WIP][PromotionBundle] The collection form type uses the new CollectionExtension

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Behat?    | no and It will use the FormContext in #2218
| Phpspec? | no
| Fixed tickets | -
| License       | MIT
| Doc PR        | -