<!--- 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 -->

'1.3.101.112': ('ED25519', ),
    '1.3.101.113': ('ED448', ),
    '1.3.111': ('ieee', ),
    '1.3.111.2.1619': ('IEEE Security in Storage Working Group', 'ieee-siswg'),
    '1.3.111.2.1619.0.1.1': ('aes-128-xts', 'AES-128-XTS'),
    '1.3.111.2.1619.0.1.2': ('aes-256-xts', 'AES-256-XTS'),
    '1.3.132': ('certicom-arc', ),
    '1.3.132.0': ('secg_ellipticCurve', ),
    '1.3.132.0.1': ('sect163k1', ),
    '1.3.132.0.2': ('sect163r1', ),
    '1.3.132.0.3': ('sect239k1', ),
    '1.3.132.0.4': ('sect113r1', ),
    '1.3.132.0.5': ('sect113r2', ),
    '1.3.132.0.6': ('secp112r1', ),
    '1.3.132.0.7': ('secp112r2', ),
    '1.3.132.0.8': ('secp160r1', ),
    '1.3.132.0.9': ('secp160k1', ),
    '1.3.132.0.10': ('secp256k1', ),
    '1.3.132.0.15': ('sect163r2', ),
    '1.3.132.0.16': ('sect283k1', ),
    '1.3.132.0.17': ('sect283r1', ),
    '1.3.132.0.22': ('sect131r1', ),
    '1.3.132.0.23': ('sect131r2', ),
    '1.3.132.0.24': ('sect193r1', ),
    '1.3.132.0.25': ('sect193r2', ),
    '1.3.132.0.26': ('sect233k1', ),
    '1.3.132.0.27': ('sect233r1', ),
    '1.3.132.0.28': ('secp128r1', ),
    '1.3.132.0.29': ('secp128r2', ),
    '1.3.132.0.30': ('secp160r2', ),