Add support for encrypted private keys

[barkbay]

ECK attempts to validate the certificates provided by the user, including the private key:

cloud-on-k8s/pkg/controller/common/certificates/pem.go

Lines 66 to 72 in 185b168

	switch {
	case block.Type == "PRIVATE KEY":
	return parsePKCS8PrivateKey(block.Bytes)
	case block.Type == "RSA PRIVATE KEY" && len(block.Headers) == 0:
	return x509.ParsePKCS1PrivateKey(block.Bytes)
	default:
	return nil, errors.New("expected PEM block to contain an RSA private key")

But the private key of the certificate might be encrypted, in that case ECK should not attempt to parse it.

We should add a case statement to detect this situation and not attempt to decode the key. IsEncryptedPEMBlock might be a good candidate to detect such a situation, with something along those lines for instance:

	switch {
	case x509.IsEncryptedPEMBlock(block):
		// Private key is encrypted, do not attempt to parse it
		return nil, nil
	case block.Type == "PRIVATE KEY":
		return parsePKCS8PrivateKey(block.Bytes)
	case block.Type == "RSA PRIVATE KEY" && len(block.Headers) == 0:
		return x509.ParsePKCS1PrivateKey(block.Bytes)
	default:
		return nil, errors.New("expected PEM block to contain an RSA private key")
	}