MC-108: Add an option to avoid generating category URL_KEY.

Bug fix: [no]
Feature addition: [yes]
Backwards compatibility break: [no]
Phpspec specification passes: [no]
Checkstyle issues: [no]*
ChangeLog updated: [no]
Fixes the following issue: The user can choose if url_key are handled by Akeneo or by Magento
Is it really used and does it work?