<!--- Verify first that your feature was not already discussed on GitHub -->
<!--- Complete *all* sections as described, this form is processed automatically -->

##### 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`

<!--- HINT: You can also paste gist.github.com links for larger files -->

except Exception as dummy:
        pass
    module.add_cleanup_file(tmp_name)  
    try:
        try:
            
            file = os.open(tmp_name, os.O_WRONLY | os.O_CREAT | os.O_TRUNC, 0o600)
            os.write(file, content)
            os.close(file)
        except Exception as e:
            try:
                os.remove(tmp_name)
            except Exception as dummy:
                pass
            module.fail_json(msg='Error while writing result into temporary file: {0}'.format(e))
        
        if os.path.exists(file_args['path']):
            module.set_fs_attributes_if_different(file_args, False)
        
        module.atomic_move(tmp_name, file_args['path'])
        
        module.set_fs_attributes_if_different(file_args, False)
    except Exception as e:
        try:
            os.remove(tmp_name)
        except Exception as dummy:
            pass
        module.fail_json(msg='Error while writing result: {0}'.format(e))
@six.add_metaclass(abc.ABCMeta)
class OpenSSLObject(object):