Add DiskUsage method to SystemApiclient

Signed-off-by: Kenfe-Mickael Laventure <mickael.laventure@gmail.com>

disk_usage.go

@@ -0,0 +1,26 @@
+package client
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"github.com/docker/docker/api/types"
+	"golang.org/x/net/context"
+)
+
+// DiskUsage requests the current data usage from the daemon
+func (cli *Client) DiskUsage(ctx context.Context) (types.DiskUsage, error) {
+	var du types.DiskUsage
+
+	serverResp, err := cli.get(ctx, "/system/df", nil, nil)
+	if err != nil {
+		return du, err
+	}
+	defer ensureReaderClosed(serverResp)
+
+	if err := json.NewDecoder(serverResp.body).Decode(&du); err != nil {
+		return du, fmt.Errorf("Error retrieving disk usage: %v", err)
+	}
+
+	return du, nil
+}