Get rid of backend parameter whenever possible (#883)

* Get rid of backend parameter whenever possible.

* Always auto-detect if backend choices are 'cryptography' and 'auto', resp. always check cryptography version.

* Improve error message.

* Update documentation.

plugins/modules/openssl_privatekey_info.py

@@ -64,6 +64,9 @@ options:
       - Determines which crypto backend to use.
       - The default choice is V(auto), which tries to use C(cryptography) if available.
       - If set to V(cryptography), will try to use the L(cryptography,https://cryptography.io/) library.
+      - Note that with community.crypto 3.0.0, all values behave the same.
+        This option will be deprecated in a later version.
+        We recommend to not set it explicitly.
     type: str
     default: auto
     choices: [auto, cryptography]
@@ -245,9 +248,8 @@ def main():
 
     result["can_load_key"] = True
 
-    backend, module_backend = select_backend(
+    module_backend = select_backend(
         module,
-        module.params["select_crypto_backend"],
         data,
         passphrase=module.params["passphrase"],
         return_private_key_data=module.params["return_private_key_data"],