[r=blake-rouse][bug=1539248][author=mpontillo] Enable IP range support for StaticIPAddress.objects.allocate_new().
* Fix StaticIPAddress.objects.allocate_new() to work from Subnet and IPRange model objects, rather than take parameters formerly found on NodeGroupInterface.
* Fix test cases which relied on the previous behavior.
* Add migration to cover minor changes to IPRange fields.
* Automatically reserve the lower 32 bits of IPv6 /64 subnets.
(for future MAAS use)