zebra: rib nb changes with redefined nexthop model

zebra: rib nb changes with redefined nexthop model

RIB operational northbound changes based on redfined
nexthop model.

Testing Done:

{
  "afi-safi-name": "ipv4-unicast",
  "table-id": 254,
  "route": [
    {
      "prefix": "0.0.0.0/0",
      "route-entry": [
        {
          "protocol": "kernel",
          "distance": 0,
          "metric": 100,
          "selected": [null],
          "installed": [null],
          "internal-flags": 8,
          "internal-status": 16,
          "uptime": "2020-06-06T00:28:26.139797Z",
          "nexthop-group": {
            "id": 15,
            "nexthop": [
              {
                "nh-type": "ip4-ifindex",
                "vrf": "0",
                "gateway": "10.0.2.2",
                "interface": "enp0s3",
                "active": [null],
                "fib": [null]
              }
            ]
          }
        }
      ]
    },
    {
      "prefix": "1.1.1.4/32",
      "route-entry": [
        {
          "protocol": "kernel",
          "distance": 0,
          "metric": 0,
          "selected": [null],
          "installed": [null],
          "internal-flags": 8,
          "internal-status": 16,
          "uptime": "2020-06-06T00:28:26.139790Z",
          "nexthop-group": {
            "id": 16,
            "nexthop": [
              {
                "nh-type": "ifindex",
                "vrf": "0",
                "gateway": "",
                "interface": "dummy4",
                "active": [null],
                "fib": [null]
              }
            ]
          }
        }
      ]
    }
  ]
}

Signed-off-by: Chirag Shah <chirag@cumulusnetworks.com>