rkt: Map kubelet's `--stage1-image` flag to rkt's `--stage1-name` flag.

This enables rkt to use cached stage1 image instead of unpacking the
stage1 image every time for every pod.

After this change, users need to preload the stage1 images in order to
enable rkt to find the stage1 image with the name specified by this flag.

cluster/gce/coreos/master-docker.yaml

@@ -122,8 +122,6 @@ coreos:
         --cluster-domain=${DNS_DOMAIN} \
         --logtostderr=true \
         --container-runtime=docker \
-        --rkt-path=/opt/rkt/rkt \
-        --rkt-stage1-image=/opt/rkt/stage1-coreos.aci \
         --configure-cbr0=${KUBERNETES_CONFIGURE_CBR0} \
         --pod-cidr=${MASTER_IP_RANGE} \
         --register-schedulable=false \

pkg/kubelet/rkt/rkt.go

@@ -1028,7 +1028,7 @@ func (r *Runtime) preparePodArgs(manifest *appcschema.PodManifest, manifestFileN
 	// 3) empty; whatever rkt's compiled to default to
 	stage1ImageCmd := ""
 	if r.config.Stage1Image != "" {
-		stage1ImageCmd = "--stage1-path=" + r.config.Stage1Image
+		stage1ImageCmd = "--stage1-name=" + r.config.Stage1Image
 	}
 	if stage1Name, ok := manifest.Annotations.Get(k8sRktStage1NameAnno); ok {
 		stage1ImageCmd = "--stage1-name=" + stage1Name

pkg/kubelet/rkt/rkt_test.go

@@ -1832,15 +1832,15 @@ func TestPreparePodArgs(t *testing.T) {
 					},
 				},
 			},
-			"stage1-path",
+			"stage1-image0",
 			[]string{"prepare", "--quiet", "--pod-manifest", "file", "--stage1-name=stage1-image"},
 		},
 		{
 			appcschema.PodManifest{
 				Annotations: appctypes.Annotations{},
 			},
-			"stage1-path",
-			[]string{"prepare", "--quiet", "--pod-manifest", "file", "--stage1-path=stage1-path"},
+			"stage1-image0",
+			[]string{"prepare", "--quiet", "--pod-manifest", "file", "--stage1-name=stage1-image0"},
 		},
 		{
 			appcschema.PodManifest{