Implements US6909 (Blocking Tenants from IPv6 Usage) Primitive temporary disabling via the APIs

Inevitably this breaks the unit tests, since there are tests such as:

 test_create_subnet_with_ipv6_address_mode

All this does is disallow 6 as a parameter when creating a subnet.

If we want to improve the UX of tenants we could add a custom message ("IPV6 Has been disabled for now") but figure this isn't important for now.