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##### SUMMARY
Allow to specify a new "format" parameter in "openssl_privatekey". Today the output format is decided by a very simple heuristic which requires further commands to work properly.
I need, for example, to generate a RSA private key but in PKCS8 format. The current heuristic uses OpenSSH format for RSA keys and only uses PKCS8 for Ed25519 and similar. This makes me use a "command" with "openssl" to convert the generated key, which leads to idempotency problems and unnecessary complexity.

##### ISSUE TYPE
- Feature Idea

##### COMPONENT NAME
openssl_privatekey

##### ADDITIONAL INFORMATION
<!--- Describe how the feature would be used, why it is needed and what it would solve -->
The current heuristic should be kept unless overwritten by user.

<!--- Paste example playbooks or commands between quotes below -->
```yaml
openssl_privatekey:
  path: /etc/ssl/private/ansible.com.pem
  format: pkcs8
```
The values allowed are `pkcs1`, `raw` and `pkcs8`

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pass
class PrivateKeyBase(crypto_utils.OpenSSLObject):
    def __init__(self, module):
        super(PrivateKeyBase, self).__init__(
            module.params['path'],
            module.params['state'],
            module.params['force'],
            module.check_mode
        )
        self.size = module.params['size']
        self.passphrase = module.params['passphrase']
        self.cipher = module.params['cipher']
        self.privatekey = None
        self.fingerprint = {}
        self.backup = module.params['backup']
        self.backup_file = None
        if module.params['mode'] is None:
            module.params['mode'] = '0600'
    @abc.abstractmethod
    def _generate_private_key_data(self):