Add explicit address family to AddressScope

Add explicit address family to AddressScope

Adds an ip_version field for enforcing an address family on
an AddressScope.  All SubnetPools in an AddressScope must be
from the same address family. A SubnetPool cannot be created
in an AddressScope with a different address family.

Change-Id: Ibc6de08e0ef58a5da954d13f274f6003012a76cd
Partially-Implements: blueprint address-scopes