The (host)name for aux-addresses should
be unique, otherwise later values overwrite
earlier values.

Before this change, the example command
would send this API request;

    {
      "Attachable": false,
      "CheckDuplicate": true,
      "Driver": "overlay",
      "EnableIPv6": false,
      "IPAM": {
        "Config": [
          {
            "Gateway": "192.168.0.100",
            "IPRange": "192.168.1.0/24",
            "Subnet": "192.168.0.0/16"
          },
          {
            "AuxiliaryAddresses": {
              "a": "192.170.1.5",
              "b": "192.170.1.6"
            },
            "Gateway": "192.170.0.100",
            "Subnet": "192.170.0.0/16"
          }
        ],
        "Driver": "default",
        "Options": {
        }
      },
      "Internal": false,
      "Labels": {
      },
      "Name": "my-multihost-network",
      "Options": {
      }
    }

After this change, the request looks
like this (all aux-addresses preserved);

    {
      "Attachable": false,
      "CheckDuplicate": true,
      "Driver": "overlay",
      "EnableIPv6": false,
      "IPAM": {
        "Config": [
          {
            "AuxiliaryAddresses": {
              "my-router": "192.168.1.5",
              "my-switch": "192.168.1.6"
            },
            "Gateway": "192.168.0.100",
            "IPRange": "192.168.1.0/24",
            "Subnet": "192.168.0.0/16"
          },
          {
            "AuxiliaryAddresses": {
              "my-printer": "192.170.1.5",
              "my-nas": "192.170.1.6"
            },
            "Gateway": "192.170.0.100",
            "Subnet": "192.170.0.0/16"
          }
        ],
        "Driver": "default",
        "Options": {
        }
      },
      "Internal": false,
      "Labels": {
      },
      "Name": "my-multihost-network",
      "Options": {
      }
    }

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>