Werk #8647: cmk-update-agent: Fixed certificate verification issues in case openssl tools are not available

When communicating via HTTPS with the update server, the cmk-update-agent script needs to verify the certificates of the server. The allowed certificates are stored at /var/lib/check_mk_agent/cas.

In this directory there need to be the certificates (.pem files) and symlinks named with the "subject hashes" of the certificates. In previous versions these symlinks were created by the c_rehash command which is not available on all plaftforms.

We have changed that now to use the python OpenSSL bindings (if available) and fallback to the c_rehash command only in case they are not available.

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