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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 _get_ec_class(self, ectype):
        ecclass = cryptography.hazmat.primitives.asymmetric.ec.__dict__.get(ectype)
        if ecclass is None:
            self.module.fail_json(msg='Your cryptography version does not support {0}'.format(ectype))
        return ecclass
    def _add_curve(self, name, ectype, deprecated=False):
        def create(size):
            ecclass = self._get_ec_class(ectype)
            return ecclass()
        def verify(privatekey):
            ecclass = self._get_ec_class(ectype)
            return isinstance(privatekey.private_numbers().public_numbers.curve, ecclass)
        self.curves[name] = {
            'create': create,
            'verify': verify,
            'deprecated': deprecated,
        }
    def __init__(self, module):
        super(PrivateKeyCryptography, self).__init__(module)
        self.curves = dict()
        self._add_curve('secp384r1', 'SECP384R1')
        self._add_curve('secp521r1', 'SECP521R1')
        self._add_curve('secp224r1', 'SECP224R1')
        self._add_curve('secp192r1', 'SECP192R1')