rest_firewall: support IPv6 match conditions

this patch gets rest_firewall to support IPv6 match conditions.

'ipv6_src' and 'ipv6_dst' are allowed as key.
'ICMPv6' is allowed as the value of 'nw_proto'.

e.g.)

  curl -X POST -d '{"ipv6_src": "10::/64"}' http://localhost:8080/firewall/rules/0000000000000001

  curl http://localhost:8080/firewall/rules/0000000000000001

  [
    {
      "access_control_list": [
        {
          "rules": [
            {
              "priority": 1,
              "dl_type": "IPv6",
              "ipv6_src": "10::/64",
              "rule_id": 1,
              "actions": "ALLOW"
            }
          ]
        }
      ],
      "switch_id": "0000000000000001"
    }
  ]