Add containers’ networks to /containers/json

After addition of multi-host networking in Docker 1.9, Docker Remote
API is still returning only the network specified during creation
of the container in the “List Containers” (`/containers/json`) endpoint:

    ...
    "HostConfig": {
      "NetworkMode": "default"
    },

The list of networks containers are attached to is only available at
Get Container (`/containers/<id>/json`) endpoint.
This does not allow applications utilizing multi-host networking to
be built on top of Docker Remote API.

Therefore I added a simple `"NetworkSettings"` section to the
`/containers/json` endpoint. This is not identical to the NetworkSettings
returned in Get Container (`/containers/<id>/json`) endpoint. It only
contains a single field `"Networks"`, which is essentially the same
value shown in inspect output of a container.

This change adds the following section to the `/containers/json`:

    "NetworkSettings": {
      "Networks": {
        "bridge": {
          "EndpointID": "2cdc4edb1ded3631c81f57966563e...",
          "Gateway": "172.17.0.1",
          "IPAddress": "172.17.0.2",
          "IPPrefixLen": 16,
          "IPv6Gateway": "",
          "GlobalIPv6Address": "",
          "GlobalIPv6PrefixLen": 0,
          "MacAddress": "02:42:ac:11:00:02"
        }
      }
    }

This is of type `SummaryNetworkSettings` type, a minimal version of
`api/types#NetworkSettings`.

Actually all I need is the network name and the IPAddress fields. If folks
find this addition too big, I can create a `SummaryEndpointSettings` field
as well, containing just the IPAddress field.

Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>

integration-cli/docker_api_containers_test.go

@@ -68,6 +68,7 @@ func (s *DockerSuite) TestContainerApiGetJSONNoFieldsOmitted(c *check.C) {
 		"Ports",
 		"Labels",
 		"Status",
+		"NetworkSettings",
 	}
 
 	// decoding into types.Container do not work since it eventually unmarshal