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
Merge branch 'master' into master-PIM-3309

Conflicts:
	src/Pim/Bundle/BaseConnectorBundle/Tests/Unit/Validator/Import/ProductImportValidatorTest.php
	src/Pim/Bundle/BaseConnectorBundle/Validator/Import/ProductImportValidator.php
Merge pull request #1743 from akeneo/master-PIM-3309

Master pim 3309