[Validator][Constraints] Update CollectionValidator using new Validator API

Bug fix: yes
Feature addition: no
Backwards compatibility break: no
Symfony2 tests pass: yes
Fixes the following tickets: #6648
License of the code: MIT

In the tests `$i = 1` is because in `validate` the first call to `$this->context` is `$this->context->getGroup();`, should I write this in a comment before each declaration?