bgpd: Print pretty json output for bgp_show_table()

bgpd: Print pretty json output for bgp_show_table()

This is not very cool:

```
{
 "vrfId": 0,
 "vrfName": "default",
 "tableVersion": 4,
 "routerId": "192.168.0.1",
 "defaultLocPrf": 100,
 "localAS": 200,
 "routes": { "10.0.0.150/32": [{"valid":true,"bestpath":true,"pathFrom":"external","prefix":"10.0.0.150","prefixLen":32,"network":"10.0.0.150\/32","med":0,"metric":0,"weight":32768,"peerId":"(unspec)","aspath":"200 200 200","path":"200 200 200","origin":"incomplete","nexthops":[{"ip":"0.0.0.0","afi":"ipv4","used":true}]}],"10.0.0.200/32": [{"valid":true,"bestpath":true,"pathFrom":"external","prefix":"10.0.0.200","prefixLen":32,"network":"10.0.0.200\/32","med":0,"metric":0,"weight":32768,"peerId":"(unspec)","aspath":"200 200 200","path":"200 200 200","origin":"incomplete","nexthops":[{"ip":"0.0.0.0","afi":"ipv4","used":true}]}],"10.0.2.0/24": [{"valid":true,"bestpath":true,"pathFrom":"external","prefix":"10.0.2.0","prefixLen":24,"network":"10.0.2.0\/24","med":0,"metric":0,"weight":32768,"peerId":"(unspec)","aspath":"200 200 200","path":"200 200 200","origin":"incomplete","nexthops":[{"ip":"0.0.0.0","afi":"ipv4","used":true}]}],"192.168.0.0/24": [{"valid":true,"bestpath":true,"pathFrom":"external","prefix":"192.168.0.0","prefixLen":24,"network":"192.168.0.0\/24","med":0,"metric":0,"weight":32768,"peerId":"(unspec)","aspath":"200 200 200","path":"200 200 200","origin":"incomplete","nexthops":[{"ip":"0.0.0.0","afi":"ipv4","used":true}]}] }  }
```

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