Validate products according to variant group axes 

Bug fix: no
Feature addition: no
Backwards compatibility break: no
Unit test passes: yes
Behat scenarios passes: ~
Checkstyle issues: no
Documentation PR: ~
Fixes the following jira:
 - PIM-1248

Todo:
- [x] Validate when saving product
- [x] Validate when adding product to a variant group
- [x] Validate during product import