[FIX] point_of_sale : fix product variant name in POS

Fix product variant name in POS

***Impacted versions:***

- 8.0
- master 

***Steps to reproduce:***

1. Go on runbot and pick up a v8.0 or a master instance
2. Sign in to the ERP (database using all modules)
3. Go to (point of sale) 
4. Click on New Session or resume an old one
5. Look for a product like (iPad Retina Display);  you should flag the product in the back to making it visible on POS

***Current output:***

 - All the variant have the same name (iPad Retina Display) which mean that we cannot distinguish between them the right one

***Expected output:***
- The output should be like that : 
- iPad Retina Display (16 GB, White)
- iPad Retina Display (16 GB, Black)
- etc ..