Allow configuring the rkt binary in a kubelet with a flag.

This is necessary because coreos comes with rkt installed, and if we want to use a different version
we need some way to avoid the default one.

pkg/kubelet/rkt/config.go

@@ -21,6 +21,8 @@ import "fmt"
 // Config stores the global configuration for the rkt runtime.
 // Run 'rkt' for more details.
 type Config struct {
+	// The absolute path to the binary, or leave empty to find it in $PATH.
+	Path string
 	// The debug flag for rkt.
 	Debug bool
 	// The rkt data directory.

pkg/kubelet/rkt/rkt.go

@@ -128,11 +128,14 @@ func New(config *Config,
 		return nil, fmt.Errorf("cannot connect to dbus: %v", err)
 	}
 
-	// Test if rkt binary is in $PATH.
-	// TODO(yifan): Use a kubelet flag to read the path.
-	rktBinAbsPath, err := exec.LookPath("rkt")
-	if err != nil {
-		return nil, fmt.Errorf("cannot find rkt binary: %v", err)
+	rktBinAbsPath := config.Path
+	if rktBinAbsPath == "" {
+		// No default rkt path was set, so try to find one in $PATH.
+		var err error
+		rktBinAbsPath, err = exec.LookPath("rkt")
+		if err != nil {
+			return nil, fmt.Errorf("cannot find rkt binary: %v", err)
+		}
 	}
 
 	rkt := &runtime{