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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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def __init__(self, module):
        super(PrivateKeyPyOpenSSL, self).__init__(module)
        if module.params['type'] == 'RSA':
            self.type = crypto.TYPE_RSA
        elif module.params['type'] == 'DSA':
            self.type = crypto.TYPE_DSA
        else:
            module.fail_json(msg="PyOpenSSL backend only supports RSA and DSA keys.")
    def _generate_private_key_data(self):
        self.privatekey = crypto.PKey()
        try:
            self.privatekey.generate_key(self.type, self.size)
        except (TypeError, ValueError) as exc:
            raise PrivateKeyError(exc)
        if self.cipher and self.passphrase:
            return crypto.dump_privatekey(crypto.FILETYPE_PEM, self.privatekey,
                                          self.cipher, to_bytes(self.passphrase))
        else:
            return crypto.dump_privatekey(crypto.FILETYPE_PEM, self.privatekey)
    def _get_fingerprint(self):