MINIFICPP-2170 Fix system certificate store usage in SSLContextService on Linux

[fgerlits]

https://issues.apache.org/jira/browse/MINIFICPP-2170

Use utils::getDefaultCAFile() in the SSLContextService to find the system CA certificate store file, as the compiled-in default value is usually wrong. If the system CA certificate store file is still not found, you can set the SSL_CERT_DIR or SSL_CERT_FILE environment variables to tell OpenSSL where the certificates are.

Also: fix the incorrect handling of InvokeHTTP::DisablePeerVerification.

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[lordgamez]

Could we document the SSL_CERT_DIR and SSL_CERT_FILE environment variable options for setting the certificate somewhere? Maybe write a test using those variables as well?

[fgerlits]

Could we document the SSL_CERT_DIR and SSL_CERT_FILE environment variable options for setting the certificate somewhere? Maybe write a test using those variables as well?

I was planning to do that, but it would take quite a bit of additional code to create automatic tests for this, as it requires that none of the files listed in utils::getDefaultCAFile() exists (so we'd have to remove it), and also we'd need to create a mechanism for setting environment variables in the MiNiFi container.

I don't think we want to encourage people to use the SSL_CERT_DIR and SSL_CERT_FILE environment variables; setting the CA Certificate property in SSLContextService or the SSL Certificate Authority property in ListenHTTP is a better option.

I have tested these manually because in case there is a production issue we need a quick temporary solution for, it's good to know we have this option.