re-use keyToTemporaryFile function

[glimchb]

today we save PSK in

opi-spdk-bridge/pkg/backend/nvme_path.go

Lines 289 to 305 in b163364

	func (s *Server) keyToTemporaryFile(pskKey []byte) (string, error) {
	keyFile, err := s.psk.createTempFile("/var/tmp", "opikey")
	if err != nil {
	log.Printf("error: failed to create file for key: %v", err)
	return "", status.Error(codes.Internal, "failed to handle key")
	}
	const keyPermissions = 0600
	if err := s.psk.writeKey(keyFile.Name(), pskKey, keyPermissions); err != nil {
	log.Printf("error: failed to write to key file: %v", err)
	removeErr := os.Remove(keyFile.Name())
	log.Printf("Delete key file after key write: %v", removeErr)
	return "", status.Error(codes.Internal, "failed to handle key")
	}
	return keyFile.Name(), nil
	}

in #398 I need to save PSK also here

opi-spdk-bridge/pkg/frontend/nvme_subsystem.go

Lines 80 to 83 in b163364

	params := spdk.NvmfSubsystemAddHostParams{
	Nqn: in.NvmeSubsystem.Spec.Nqn,
	Host: in.NvmeSubsystem.Spec.Hostnqn,
	}

Let's move keyToTemporaryFile function to utils so we can re-use it