Avoid using encryptor for decrypting configuration: auth/password

[tuyennn]

Set config_path from under xml configuration field and use

type="obscure",
backend_model="Magento\Config\Model\Config\Backend\Encrypted"

Instead injecting a Magento\Framework\Encryption\EncryptorInterface
https://github.com/mobilpay/Magento/blob/master/app/code/Netopia/Netcard/Block/Qrcode.php#L157