I found the solution:

The answer is in the website_sale modul.

website_sale/controllers/main.py
(row 447-450)

    mandatory_billing_fields = ["name", "phone", "email", "street2", "city", "country_id"]

    optional_billing_fields = ["street", "state_id", "vat", "vat_subjected", "zip"]

    mandatory_shipping_fields = ["name", "phone", "street", "city", "country_id"]

    optional_shipping_fields = ["state_id", "zip"]

My team created a Modul to change the mandatory fields and we will put it online soon.

But for now this should help. 

Edit: Here is the module:
https://github.com/KontorConsulting/Odoo-8-Modules/tree/master/website_sale_mandatory_mod