IPv6 Support for TripleO Networking

This Blueprint is for adding the capability to use IPv6 native addressing on Overcloud nodes deployed using TripleO. This will be done using Heat templates that model network resources using Neutron networks and ports (similar to how this is done using IPv4 addresses currently). The values for endpoints, VIP addresses, and hostnames will all be IPv6 addresses. These values will used in nested stacks and passed to the OpenStack Puppet Modules via Hieradata files. The goal would be to support IPv4 and IPv6 simultaneously, but initial implementation may focus on choosing either IPv6 or IPv4. It will be necessary for some operators to use IPv6 only, as some datacenters are becoming IPv6 only, especially those that serve the needs of mobile devices.

Note that the work items related to dual-stack have been postponed until Pike. This will prevent having to refactor for dual-stack, and then having to refactor those changes yet again to support spine-and-leaf. This will allow refactoring work to support spine-and-leaf routed networking to happen in conjunction.