PIM-3530 : add a first version for variant group values import
PIM-3530 : re-work the group saver to copy variant group values and add
specs
PIM-3530 : re-work the product template manager and add specs
PIM-3530 : rename processor and writer to avoid to expose the concept of
product template
PIM-3530 : update the processor to erase the template values during
import and deal with validation
PIM-3530 : add a detacher interface
PIM-3530 : inject FQCN in processor
PIM-3530 : add a first behat on variant group values import
PIM-3530 : add a ProductTemplateManagerInterface
PIM-3530 : re-work ObjectDetacher and add specs
PIM-3530 : fix CS
PIM-3530 : rename object detacher service
PIM-3530 : fix delete import scenario
Merge pull request #1921 from akeneo/variant-group-template-import

PIM-3530 : variant group values import (first version)