`host_network` should support floating/virtual IP addresses

In some clusters IP addresses are dynamically assigned and reassigned to different hosts. The initial version of Multi-Host Interface networks only supports discovery of addresses that are bound to an interface when Nomad is started. This is because the network fingerprinter is only ran once on startup.

Unfortunately the problem is more nuanced than just periodically fingerprinting network interfaces. In some cases the floating address is never assigned to an interface and is managed by other networking tools and products. For example, GCP load balancers can use direct routing where the incoming packet's destination address is unchanged and is the address of the GCP managed loadbalancer and thus would never be fingerprinted by Nomad.

Nomad `host_network` configuration should support a way of configuring a virtual address that can be reported to the servers and used in scheduling.