zebra: Print json output for show_route_summary_cmd

zebra: Print json output for show_route_summary_cmd

```
spine1-debian-9# sh ip route summary json
{
  "routes":[
    {
      "fib":1,
      "rib":1,
      "type":"kernel"
    },
    {
      "fib":11,
      "rib":11,
      "type":"connected"
    },
    {
      "fib":2,
      "rib":2,
      "type":"ebgp"
    },
    {
      "fib":1,
      "rib":1,
      "type":"ibgp"
    }
  ],
  "routesTotal":15,
  "routesTotalFib":15
}
spine1-debian-9# sh ipv6 route summary json
{
  "routes":[
    {
      "fib":7,
      "rib":7,
      "type":"connected"
    }
  ],
  "routesTotal":7,
  "routesTotalFib":7
}
```

Signed-off-by: Donatas Abraitis <donatas.abraitis@gmail.com>