Allow sharing of additional IP addresses

Neutron currently lacks a way to share an IP between multiple ports to support HA active-passive application configurations. Some solutions, like load balancers, solve active-active configurations, but this requires additional resources and also creates a single-point-of-failure scenario, so HA still isn't solved.

This blueprint and accompanying spec is intended to introduce an 'enabled' flag that will be set to True for the first v4 and v6 IP on a Port, and False for all additional IPs afterwards. This flag will then be handled by Nova, and any other dependent services, to omit network configurations for disabled IPs on the guest. This will then allow any additional IP to be shared between Ports and therefore instances. This way, the user may configure whichever instance they want to take ownership of the IP without the risk of the other trying to hijack the IP on restart, or any command that would cause the undesired guest to try and take ownership of the shared IP without the user orchestrating it. We would also introduce a read-only 'shareable' attribute for the fixed_ips elements to indicate which IPs on the port can be shared to others within the same network segment.