PIM-3069: Fix image file prefixes not well generated on product creation...

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | Y
| CI currently passes? | Y
| Tests pass?          | Y
| Scenarios pass?      | ?
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | Y
| Fixed tickets        | PIM-3069
| DB schema updated?   | N
| Migration script?    | N
| Doc PR               | N

Use $product->getId() on creation (import and fixtures for example) returns null. We now use uniqid() to generate the "uniq" part of the file name.