Allows to use versions like 1.6.4 instead v1.6.4

As part of issue kubernetes/kubeadm#292 discussion, it
turned out that for users it is not always obvious that
version specification parameter must be in form "vX.Y.Z".
This patch allows to specify it in form "X.Y.Z" and
converts it internally to normal semantic version which
expected in the rest of the code.

cmd/kubeadm/app/util/version.go

@@ -26,7 +26,7 @@ import (
 
 var (
 	kubeReleaseBucketURL  = "https://storage.googleapis.com/kubernetes-release/release"
-	kubeReleaseRegex      = regexp.MustCompile(`^v(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)([-0-9a-zA-Z_\.+]*)?$`)
+	kubeReleaseRegex      = regexp.MustCompile(`^v?(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)\.(0|[1-9][0-9]*)([-0-9a-zA-Z_\.+]*)?$`)
 	kubeReleaseLabelRegex = regexp.MustCompile(`^[[:lower:]]+(-[-\w_\.]+)?$`)
 )
 
@@ -49,7 +49,10 @@ var (
 //  latest-1.0  (and similarly 1.1, 1.2, 1.3, ...)
 func KubernetesReleaseVersion(version string) (string, error) {
 	if kubeReleaseRegex.MatchString(version) {
-		return version, nil
+		if strings.HasPrefix(version, "v") {
+			return version, nil
+		}
+		return "v" + version, nil
 	} else if kubeReleaseLabelRegex.MatchString(version) {
 		url := fmt.Sprintf("%s/%s.txt", kubeReleaseBucketURL, version)
 		resp, err := http.Get(url)

cmd/kubeadm/app/util/version_test.go

@@ -45,6 +45,7 @@ func TestValidVersion(t *testing.T) {
 		"v1.6.0-alpha.0.536+d60d9f3269288f",
 		"v1.5.0-alpha.0.1078+1044b6822497da-pull",
 		"v1.5.0-alpha.1.822+49b9e32fad9f32-pull-gke-gci",
+		"v1.6.1_coreos.0",
 	}
 	for _, s := range validVersions {
 		ver, err := KubernetesReleaseVersion(s)
@@ -61,8 +62,9 @@ func TestValidVersion(t *testing.T) {
 func TestInvalidVersion(t *testing.T) {
 	invalidVersions := []string{
 		"v1.3",
-		"1.4.0",
-		"1.4.5+git",
+		"1.4",
+		"b1.4.0",
+		"c1.4.5+git",
 		"something1.2",
 	}
 	for _, s := range invalidVersions {
@@ -77,6 +79,24 @@ func TestInvalidVersion(t *testing.T) {
 	}
 }
 
+func TestValidConvenientForUserVersion(t *testing.T) {
+	validVersions := []string{
+		"1.4.0",
+		"1.4.5+git",
+		"1.6.1_coreos.0",
+	}
+	for _, s := range validVersions {
+		ver, err := KubernetesReleaseVersion(s)
+		t.Log("Valid: ", s, ver, err)
+		if err != nil {
+			t.Errorf("KubernetesReleaseVersion unexpected error for version %q: %v", s, err)
+		}
+		if ver != "v"+s {
+			t.Errorf("KubernetesReleaseVersion should return semantic version string. %q vs. %q", s, ver)
+		}
+	}
+}
+
 func TestVersionFromNetwork(t *testing.T) {
 	type T struct {
 		Content       string