PIM-3309: Validation on product values unicity

*Behats have exactly the same values as descripbed in the github issue #1657
*Behats to check the uniqueness of product's sku
*Added specs
*Fix uniqueness check on file only
*Fix uniqueness check from file to DB
Merge pull request #1727 from akeneo/PIM-3309

PIM-3309: Fix check on product unicity