Never version of Symfony::Config don't allow multiple root elements like we use for the A/B test campaign definitions. Instead, a configuration must have one root element.

Acceptance criteria:

• Campaign definition files have one root element
• The CampaignConfigurationclass doesn't call the deprecated root method any more.

Notes:
Let's take the opportunity to structure the configuration a bit more, adding the ability to add metadata outside of campaigns if needed (e.g. for ignored buckets like the one in CampaignUtilizationValidator). I suggest a structure like:

bucket_tests:
  campaigns:
    campaign_name1:
     ....
    campaign_name2:
    ....
  ignored_buckets:
   - 'foo'