PIM-2345 : clear the object manager during CSV export to avoid memory
leak
Merge pull request #1634 from akeneo/PIM-3245

PIM-2345 : clear the object manager during CSV export to avoid memory
PIM-3247: Can't display all variant groups on grid

* Improved behats
* updated CHANGELOG
* Fix bug on grid, variant groups were not correctly displayed. (max
result was applied to the joins and not directly on the variant group)
Merge pull request #1641 from akeneo/PIM-3247

PIM-3247: Can't display all variant groups on grid
Merge branch '1.2' into fix/PIM-3244