kubeadm-upgrade: small improvements to diff

1) Store the io.Writer and pass it to sub-commands in upgrade.go
2) Check if the manifest path is an empty string in diff.go:runDiff()
3) Use the io.Writer that upgrade.go defines instead of writing to
os.Stdout directly.

cmd/kubeadm/app/cmd/upgrade/diff.go

@@ -17,8 +17,8 @@ limitations under the License.
 package upgrade
 
 import (
+	"fmt"
 	"io/ioutil"
-	"os"
 
 	"github.com/golang/glog"
 	"github.com/pmezard/go-difflib/difflib"
@@ -119,6 +119,9 @@ func runDiff(flags *diffFlags, args []string) error {
 		if err != nil {
 			return err
 		}
+		if path == "" {
+			return fmt.Errorf("empty manifest path")
+		}
 		existingManifest, err := ioutil.ReadFile(path)
 		if err != nil {
 			return err
@@ -133,7 +136,7 @@ func runDiff(flags *diffFlags, args []string) error {
 			Context:  flags.contextLines,
 		}
 
-		difflib.WriteUnifiedDiff(os.Stdout, diff)
+		difflib.WriteUnifiedDiff(flags.parent.out, diff)
 	}
 	return nil
 }

cmd/kubeadm/app/cmd/upgrade/upgrade.go

@@ -37,6 +37,7 @@ type cmdUpgradeFlags struct {
 	skipPreFlight             bool
 	ignorePreflightErrors     []string
 	ignorePreflightErrorsSet  sets.String
+	out                       io.Writer
 }
 
 // NewCmdUpgrade returns the cobra command for `kubeadm upgrade`
@@ -50,6 +51,7 @@ func NewCmdUpgrade(out io.Writer) *cobra.Command {
 		printConfig:               false,
 		skipPreFlight:             false,
 		ignorePreflightErrorsSet:  sets.NewString(),
+		out: out,
 	}
 
 	cmd := &cobra.Command{