Move remaining cert helper functions to client-go/util/cert

- Move public key functions to client-go/util/cert
- Move pki file helper functions to client-go/util/cert
- Standardize on certutil package alias
- Update dependencies to client-go/util/cert

Kubernetes-commit: aac4d5382d8ea632360a08369f5adfdebce7c2c3

util/cert/io.go

@@ -138,13 +138,27 @@ func CertsFromFile(file string) ([]*x509.Certificate, error) {
 // PrivateKeyFromFile returns the private key in rsa.PrivateKey or ecdsa.PrivateKey format from a given PEM-encoded file.
 // Returns an error if the file could not be read or if the private key could not be parsed.
 func PrivateKeyFromFile(file string) (interface{}, error) {
-	pemBlock, err := ioutil.ReadFile(file)
+	data, err := ioutil.ReadFile(file)
 	if err != nil {
 		return nil, err
 	}
-	key, err := ParsePrivateKeyPEM(pemBlock)
+	key, err := ParsePrivateKeyPEM(data)
 	if err != nil {
-		return nil, fmt.Errorf("error reading %s: %v", file, err)
+		return nil, fmt.Errorf("error reading private key file %s: %v", file, err)
 	}
 	return key, nil
 }
+
+// PublicKeysFromFile returns the public keys in rsa.PublicKey or ecdsa.PublicKey format from a given PEM-encoded file.
+// Reads public keys from both public and private key files.
+func PublicKeysFromFile(file string) ([]interface{}, error) {
+	data, err := ioutil.ReadFile(file)
+	if err != nil {
+		return nil, err
+	}
+	keys, err := ParsePublicKeysPEM(data)
+	if err != nil {
+		return nil, fmt.Errorf("error reading public key file %s: %v", file, err)
+	}
+	return keys, nil
+}