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/doc_fragments/module_certificate.py

@@ -71,6 +71,9 @@ 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]

plugins/doc_fragments/module_csr.py

@@ -229,6 +229,9 @@ 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]

plugins/doc_fragments/module_privatekey.py

@@ -78,6 +78,9 @@ 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]