kubeadm: allow creating a cluster with ECDSA keys

The selected key type is defined by kubeadm's --feature-gates option:
if it contains PublicKeysECDSA=true then ECDSA keys will be generated
and used.

By default RSA keys are used still.

Signed-off-by: Dmitry Rozhkov <dmitry.rozhkov@linux.intel.com>

cmd/kubeadm/app/apis/kubeadm/BUILD

@@ -19,6 +19,7 @@ go_library(
     ],
     importpath = "k8s.io/kubernetes/cmd/kubeadm/app/apis/kubeadm",
     deps = [
+        "//cmd/kubeadm/app/features:go_default_library",
         "//staging/src/k8s.io/api/core/v1:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/apis/meta/v1:go_default_library",
         "//staging/src/k8s.io/apimachinery/pkg/runtime:go_default_library",

cmd/kubeadm/app/apis/kubeadm/types.go

@@ -17,8 +17,11 @@ limitations under the License.
 package kubeadm
 
 import (
+	"crypto/x509"
+
 	v1 "k8s.io/api/core/v1"
 	metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
+	"k8s.io/kubernetes/cmd/kubeadm/app/features"
 
 	"k8s.io/apimachinery/pkg/runtime/schema"
 )
@@ -400,6 +403,15 @@ func (cfg *ClusterConfiguration) GetControlPlaneImageRepository() string {
 	return cfg.ImageRepository
 }
 
+// PublicKeyAlgorithm returns the type of encryption keys used in the cluster.
+func (cfg *ClusterConfiguration) PublicKeyAlgorithm() x509.PublicKeyAlgorithm {
+	if features.Enabled(cfg.FeatureGates, features.PublicKeysECDSA) {
+		return x509.ECDSA
+	}
+
+	return x509.RSA
+}
+
 // HostPathMount contains elements describing volumes that are mounted from the
 // host.
 type HostPathMount struct {