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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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'''
    Given a list of constraints, returns a tuple (ca, path_length).
    Raises an OpenSSLObjectError if a constraint is unknown or cannot be parsed.
    '''
    ca = False
    path_length = None
    if constraints:
        for constraint in constraints:
            if constraint.startswith('CA:'):
                if constraint == 'CA:TRUE':
                    ca = True
                elif constraint == 'CA:FALSE':
                    ca = False
                else:
                    raise OpenSSLObjectError('Unknown basic constraint value "{0}" for CA'.format(constraint[3:]))
            elif constraint.startswith('pathlen:'):
                v = constraint[len('pathlen:'):]
                try:
                    path_length = int(v)
                except Exception as e:
                    raise OpenSSLObjectError('Cannot parse path length constraint "{0}" ({1})'.format(v, e))
            else:
                raise OpenSSLObjectError('Unknown basic constraint "{0}"'.format(constraint))
    return ca, path_length
def binary_exp_mod(f, e, m):