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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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if self.backup:
            self.backup_file = module.backup_local(self.path)
        super(PrivateKeyBase, self).remove(module)
    @abc.abstractmethod
    def _check_passphrase(self):
        pass
    @abc.abstractmethod
    def _check_size_and_type(self):
        pass
    def check(self, module, perms_required=True):
        """Ensure the resource is in its desired state."""
        state_and_perms = super(PrivateKeyBase, self).check(module, perms_required)
        if not state_and_perms or not self._check_passphrase():
            return False
        return self._check_size_and_type()
    def dump(self):
        """Serialize the object into a dictionary."""
        result = {
            'size': self.size,
            'filename': self.path,
            'changed': self.changed,
            'fingerprint': self.fingerprint,
        }
        if self.backup_file:
            result['backup_file'] = self.backup_file
        return result
class PrivateKeyPyOpenSSL(PrivateKeyBase):