bgpd: Show the instance name when displaying statistics

bgpd: Show the instance name when displaying statistics

```
exit1-debian-9# sh ip bgp vrf all statistics json
{
  "ipv4Unicast":[
    {
      "instance":"VRF default",
      "totalAdvertisements":2,
      "totalPrefixes":2,
      "averagePrefixLength":24,
      "unaggregateablePrefixes":2,
      "maximumAggregateablePrefixes":0,
      "bgpAggregateAdvertisements":0,
      "addressSpaceAdvertised":512,
      "%announced":1.1920928955078125e-05,
      "\/8equivalent":3.0517578125e-05,
      "\/24equivalent":2,
      "advertisementsWithPaths":2,
      "longestAsPath":0,
      "averageAsPathLengthHops":0,
      "largestAsPath":0,
      "averageAsPathSizeBytes":0,
      "highestPublicAsn":0
    },
    {
      "instance":"VRF testas",
      "totalAdvertisements":0,
      "totalPrefixes":0,
      "averagePrefixLength":0,
      "unaggregateablePrefixes":0,
      "maximumAggregateablePrefixes":0,
      "bgpAggregateAdvertisements":0,
      "addressSpaceAdvertised":0,
      "%announced":0,
      "\/8equivalent":0,
      "\/24equivalent":0,
      "advertisementsWithPaths":0,
      "longestAsPath":0,
      "averageAsPathLengthHops":0,
      "largestAsPath":0,
      "averageAsPathSizeBytes":0,
      "highestPublicAsn":0
    }
  ]
}
```

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