bgpd: Fix memberCount bgp peer-grp json output

memberCount object is moved to outer dict as better not to add into
existing collections.

Supported commands:

```
show bgp peer-group json ==> default vrf
show bgp vrf tenant1 peer-group json => non default vrf
```

Before Fix:

```
leaf-11# show bgp peer-group json
{
  "ISL":{
    "remoteAs":0,
    "type":"external",
    "addressFamiliesConfigured":[
      "IPv4 Unicast"
    ],
    "members":{
      "swp1":{
        "status":"Established"
      },
      "swp2":{
        "status":"Established"
      },
      "swp3":{
        "status":"Established"
      },
      "swp4":{
        "status":"Established"
      },
      "memberCount":4
    }
  }
}
leaf-11#
```

```
superspine1# show bgp vrf tenant1 peer-group json
{
  "vrfpeers":{
    "remoteAs":0,
    "type":"external",
    "addressFamiliesConfigured":[
      "IPv4 Unicast",
      "IPv6 Unicast"
    ],
    "dynamicRanges":{
      "IPv4":{
        "count":1,
        "ranges":[
          "0.0.0.0/0"
        ]
      },
      "IPv6":{
        "count":1,
        "ranges":[
          "::/0"
        ]
      }
    },
    "members":{
      "101.101.1.1":{
        "status":"Established",
        "dynamic":true
      },
      "memberCount":1
    }
  }
}
```

After fix:

```
leaf-11# show bgp peer-group json
{
  "ISL":{
    "remoteAs":0,
    "type":"external",
    "addressFamiliesConfigured":[
      "IPv4 Unicast"
    ],
    "memberCount":4,
    "members":{
      "swp1":{
        "status":"Established"
      },
      "swp2":{
        "status":"Established"
      },
      "swp3":{
        "status":"Established"
      },
      "swp4":{
        "status":"Established"
      }
    }
  }
}
leaf-11#
```

```
superspine1# show bgp vrf tenant1 peer-group json
{
  "vrfpeers":{
    "remoteAs":0,
    "type":"external",
    "addressFamiliesConfigured":[
      "IPv4 Unicast",
      "IPv6 Unicast"
    ],
    "dynamicRanges":{
      "IPv4":{
        "count":1,
        "ranges":[
          "0.0.0.0/0"
        ]
      },
      "IPv6":{
        "count":1,
        "ranges":[
          "::/0"
        ]
      }
    },
    "memberCount":1,
    "members":{
      "101.101.1.1":{
        "status":"Established",
        "dynamic":true
      }
    }
  }
}
```

UT done

Ticket:#4152925

Issue: 4152925

Signed-off-by: Sindhu Parvathi Gopinathan's <sgopinathan@nvidia.com>

bgpd/bgp_vty.c

@@ -17029,8 +17029,8 @@ static int bgp_show_one_peer_group(struct vty *vty, struct peer_group *group,
 			member_count++;
 		}
 		if (json) {
-			json_object_int_add(json_peer_group_members,
-					    "memberCount", member_count);
+			json_object_int_add(json_peer_group, "memberCount",
+					    member_count);
 			json_object_object_add(json_peer_group, "members",
 					       json_peer_group_members);
 		}