* New cryptography backend for openssl_certificate
load_* functions in module_utils/crypto.py now have a backend paramter
which when set to 'cryptography' will return cryptography objects so
they can be used for both pyopenssl and cryptography backends.
Added a select_message_digest function too returning a cryptography
digest hash from `cryptography.hazmat.primitives.hashes`
Added new classes for Cryptography backend
* Run test with various backends.
* Prefixing tests.
* Make sure we have the correct backend available.
* Linting (flake8).
* Moved cryptography import to separate try/except
* Make sure certificate is actually valid at some time in the past.
* Improve error handling.
* Trying to fix validation for cryptography backend.
* Fixed issue with keyUsage test in assertonly
* Fixed CI/Lint issues
* Fix private key problem for OwnCA.
* Cryptography backend doesn't support v2 certs.
* issue an expired cert with command when using cryptography backend
* Added warning when backend is auto and v2 cert is requested
* Bumped min cryptography version to 1.6
* Correctly check for failure when backend is cryptography and cert is v2
* Use self.backend where possible
* Use secp521r1 EC when testing on CentOS6
* Fixed pylint issue
* AcmeCertificate support for both backends
* Review fixes
* Fixed missing '(' when raising error
* Fixed date_fmt loop
* Updated docs and requirements with cryptography
* Add openssl_certificate to changelog.
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