bridge: this patch adds json support for bridge mdb show

bridge: this patch adds json support for bridge mdb show

This patch adds json output to bridge mdb show

Normal Output:
$ bridge -d -s mdb show
dev br0 port swp3 grp 239.0.0.1 temp  vid 128 172.26
dev br0 port swp3 grp 239.0.0.1 temp  vid 64 172.26
dev br0 port swp2 grp 239.0.0.2 temp  vid 1024 172.26
dev br0 port swp2 grp 239.0.0.2 temp  vid 256 172.26
dev br0 port swp2 grp 239.0.0.2 temp  vid 1 172.26
dev br0 port swp3 grp 239.0.0.1 temp  vid 1 172.26
router ports on br0: swp4    0.00 permanent
router ports on br0: swp5    0.00 permanent

Json Output:
$ bridge -d -s -j mdb show
{
    "mdb": [{
            "dev": "br0",
            "port": "swp3",
            "grp": "239.0.0.1",
            "state": "temp",
            "vid": 128,
            "timer": " 166.74"
        },{
            "dev": "br0",
            "port": "swp3",
            "grp": "239.0.0.1",
            "state": "temp",
            "vid": 64,
            "timer": " 166.74"
        },{
            "dev": "br0",
            "port": "swp2",
            "grp": "239.0.0.2",
            "state": "temp",
            "vid": 1024,
            "timer": " 166.74"
        },{
            "dev": "br0",
            "port": "swp2",
            "grp": "239.0.0.2",
            "state": "temp",
            "vid": 256,
            "timer": " 166.74"
        },{
            "dev": "br0",
            "port": "swp2",
            "grp": "239.0.0.2",
            "state": "temp",
            "vid": 1,
            "timer": " 166.74"
        },{
            "dev": "br0",
            "port": "swp3",
            "grp": "239.0.0.1",
            "state": "temp",
            "vid": 1,
            "timer": " 166.74"
        }
    ],
    "router": {
        "br0": [{
                "port": "swp4",
                "timer": "   0.00",
                "type": "permanent"
            },{
                "port": "swp5",
                "timer": "   0.00",
                "type": "permanent"
            }
        ]
    }
}

Signed-off-by: Nikhil Gajendrakumar <nikhil@cumulusnetworks.com>
Signed-off-by: Roopa Prabhu <roopa@cumulusnetworks.com>