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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`

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

from __future__ import absolute_import, division, print_function
__metaclass__ = type
ANSIBLE_METADATA = {'metadata_version': '1.1',
                    'status': ['preview'],
                    'supported_by': 'community'}
DOCUMENTATION = r'''
---
module: openssl_privatekey
version_added: "2.3"
short_description: Generate OpenSSL private keys
description:
    - This module allows one to (re)generate OpenSSL private keys.
    - One can generate L(RSA,https://en.wikipedia.org/wiki/RSA_(cryptosystem)),
      L(DSA,https://en.wikipedia.org/wiki/Digital_Signature_Algorithm),
      L(ECC,https://en.wikipedia.org/wiki/Elliptic-curve_cryptography) or
      L(EdDSA,https://en.wikipedia.org/wiki/EdDSA) private keys.
    - Keys are generated in PEM format.