[WIP] PIM-1771 product repository

- [ ] FlexibleEntityRepositoryInterface should not expose QueryBuilder (to prepare Mongo impl)

Bug fix: [yes|no]
Feature addition: [yes|no]
Backwards compatibility break: [yes|no]
Unit test passes: [yes|no]
Behat scenarios passes: [yes|no]
Checkstyle issues: [yes|no]*
ChangeLog updated: [yes|no]
Documentation PR: [link to the documentation PR if there is one]
Fixes the following jira: PIM-1771