PIM-2105 Check the uniqueness of values for unique attributes during product import 

| Q                   | A
| ------------------- | ---
| Bug fix?            | Y
| New feature?        | N 
| BC breaks?          | N
| Tests pass?         | Y
| Scenarios pass?     | Y
| Checkstyle issues?  | N
| Changelog updated?  | N
| Fixed tickets       | PIM-2105
| Doc PR              |