OS-438, IPv6 support for Infoblox

* In https://jira.inca.infoblox.com:8443/browse/OPENSTACK-438 you may grab shell script to test functionality.
* Currently we support only DUID-LL stateful IPv6 (https://tools.ietf.org/html/rfc3315#section-9.4), so to get IPv6 in stateful mode run dhclient -6 -D LL eth0

You seem to be fixing an issue which must be fixed in `NeutronCorePluginV2._generate_ip()`. `_generate_ip()` must always return the same type, you should not check it in the calling code.