API: Proxy neutron configuration to guest instance (partial)

Currently the metadata service only contains minimal network information. Like user_data and vendor_data, we should also provide network_data

Really we should provide all the information we can (from neutron) in both config drive and metadata service.

Main use case:
* cloud has no DHCP
* image has no "nova-agent" or similar
* but image is still able to setup network info
* image can have custom startup scripts to get networking config from Config Drive

Other usecase:
consider a VM with the first interface configured by DHCP, and all other interfaces on private networks where the interfaces are statically configured, but you are not using config drive, just the metadata service, and not cheating by doing file injection, presenting the data in a guest agnostic format.

More discussion from Icehouse summit is here:
https://etherpad.openstack.org/IcehouseNovaMetadataService

Notes:
* only in v3 API
* information will only be populated if neutron is enabled
* information comes from current network_info for instance
* API should be fairly similar to the neutron get port details API
* but we only really need concrete info: mac address, fixed IP address, subnet, gateway, host routes, etc
* but probably best to include at least: neutron port-id, neutron network-id, neutron subnet-id
* it should all be grouped by mac address, so guest can link it to the VIFs correctly

Sample API for getting network information from metadata service

GET: http://169.254.169.254/openstack/latest/metadata/network_data

JSON Response

{
   "ports":[
      {
         "name":"foo",
         "network_id":"ebda9658-093b-41ba-80ce-0cf8cb8365d4",
         "mac_address":"fa:16:3e:b9:ef:05",
         "fixed_ips":[
            {
                "subnet-id":"91e47a57-7508-46fe-afc9-fc454e8580e1",
               "ip_address":"172.24.4.227",
               "cidr":"10.0.3.0/24",
               "dns_nameservers":["10.0.3.2"],
               "enable_dhcp":true,
               "gateway_ip":"10.0.3.1",
               "host_routes":[{"destination":"10.0.3.0/24", "nexthop":"172.24.4.228"}],
               "ip_version":4,
            }
         ],
         "id":"664ebd1a-facd-4c20-948c-07a784475ab0",
      },
      {
         "name":"",
         "network_id":...
         ...
      }
   ]
}