PIM-2662 : introduce an abstract completeness model
PIM-2662 : update mongo completeness normalizer
PIM-2662 : update changelog
PIM-2662 : fix the completeness join to make it work with abstract
completeness
PIM-2662 : fix the product repository to be able to join on abstract
completeness
Merge pull request #1151 from akeneo/feature/abstract_completeness

Abstract completeness (extensibility point)