ifupdown: iface: fix changes in default json output and ifquery -r output for inet6

ifupdown: iface: fix changes in default json output and ifquery -r output for inet6

Ticket: CM-12370
Reviewed By: Roopa, Nikhil G
Testing Done:

    incremental commit of:
    bef23f40a1e5 dhcp: add support for inet + inet6 dhcp on same interface

    (jessie-30-dev-switch-amd64-sbuild)root@host1:/home/cumulus# ifquery swp1 -o json
    [
        {
            "name": "swp1",
            "addr_method": "dhcp",
            "addr_family": "inet",
            "auto": true,
            "config": {
                "link-duplex": "full",
                "link-autoneg": "off",
                "link-speed": "10000"
            }
        },
        {
            "name": "swp1",
            "addr_method": "dhcp",
            "addr_family": "inet6",
            "auto": true,
            "config": {}
        }
    ]
    (jessie-30-dev-switch-amd64-sbuild)root@host1:/home/cumulus# ifquery swp1 -o json -c
    [
      {
        "name": "swp1",
        "addr_method": "dhcp",
        "addr_family": "inet",
        "auto": true,
        "config": {},
        "config_status": {},
        "status": "pass"
      },
      {
        "name": "swp1",
        "addr_method": "dhcp",
        "addr_family": "inet6",
        "auto": true,
        "config": {},
        "config_status": {},
        "status": "pass"
      }
    ]
    (jessie-30-dev-switch-amd64-sbuild)root@host1:/home/cumulus# ifquery swp1 -o json -r
    [
      {
        "name": "swp1",
        "addr_method": "dhcp",
        "addr_family": [
          "inet",
          "inet6"
        ],
        "auto": true,
        "config": {}
      }
    ]
    (jessie-30-dev-switch-amd64-sbuild)root@host1:/home/cumulus#

Signed-off-by: Julien Fortin <julien@cumulusnetworks.com>