PIM-2613 : prefer interfaces than abstract classes for models

 - [x] AbstractProduct -> ProductInterface
 - [x] AbstractProductValue -> ProductValueInterface
 - [x] AbstractProductPrice -> ProductPriceInterface
 - [x] AbstractMetric -> MetricInterface
 - [x] AbstractCompleteness -> CompletenessInterface

| Q                    | A
| -------------------- | ---
| Bug fix?             | N
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-2613
| DB schema updated?   |
| Migration script?    |
| Doc PR               |