Fix sku in image file name problem

| Q                   | A
| ------------------- | ---
| Bug fix?            | Y
| New feature?        | N
| BC breaks?          | N
| Tests pass?         | Y
| Scenarios pass?     | Y
| Checkstyle issues?  | N
| Changelog updated?  | Y
| Fixed tickets       | PIM-2728
| Doc PR              |
Well MongoId "is a 12-byte value consisting of a 4-byte timestamp, a 3-byte
machine id, a 2-byte process id, and a 3-byte counter" (
http://api.mongodb.org/ruby/current/BSON/ObjectId.html).

So there's a good chance if you take the 6-8 first characters, you'll get
the timestamp (which is not that unique)