server: add auth info to image pull

Fix the following upstream k8s's e2e-node test:
```
should be able to pull from private registry with secret [Conformance]
```

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

server/image_pull.go

@@ -1,8 +1,12 @@
 package server
 
 import (
+	"encoding/base64"
+	"strings"
+
 	"github.com/Sirupsen/logrus"
 	"github.com/containers/image/copy"
+	"github.com/containers/image/types"
 	"golang.org/x/net/context"
 	pb "k8s.io/kubernetes/pkg/kubelet/api/v1alpha1/runtime"
 )
@@ -17,7 +21,32 @@ func (s *Server) PullImage(ctx context.Context, req *pb.PullImageRequest) (*pb.P
 	if img != nil {
 		image = img.Image
 	}
+	var (
+		username string
+		password string
+	)
+	if req.GetAuth() != nil {
+		username = req.GetAuth().Username
+		password = req.GetAuth().Password
+		if req.GetAuth().Auth != "" {
+			var err error
+			username, password, err = decodeDockerAuth(req.GetAuth().Auth)
+			if err != nil {
+				return nil, err
+			}
+		}
+	}
 	options := &copy.Options{}
+	// a not empty username should be sufficient to decide whether to send auth
+	// or not I guess
+	if username != "" {
+		options.SourceCtx = &types.SystemContext{
+			DockerAuthConfig: &types.DockerAuthConfig{
+				Username: username,
+				Password: password,
+			},
+		}
+	}
 	_, err := s.images.PullImage(s.imageContext, image, options)
 	if err != nil {
 		return nil, err
@@ -28,3 +57,18 @@ func (s *Server) PullImage(ctx context.Context, req *pb.PullImageRequest) (*pb.P
 	logrus.Debugf("PullImageResponse: %+v", resp)
 	return resp, nil
 }
+
+func decodeDockerAuth(s string) (string, string, error) {
+	decoded, err := base64.StdEncoding.DecodeString(s)
+	if err != nil {
+		return "", "", err
+	}
+	parts := strings.SplitN(string(decoded), ":", 2)
+	if len(parts) != 2 {
+		// if it's invalid just skip, as docker does
+		return "", "", nil
+	}
+	user := parts[0]
+	password := strings.Trim(parts[1], "\x00")
+	return user, password, nil
+}