Support for passphrase protected certificate keys

Maxscale should support TLS certificates with password / passphrase protected keys.

I haven't tested how it would deal with such keys, but I assume it would "suffer" from the same TLS library default behavior as the MariaDB server and would try to prompt the user for passwords / passphrases, so breaking automated service startups when faced with such keys.

What makes things more complicated here is that unlike the server Maxscale does not only have to deal with a single certificate, but can have different certificates per server, listener, and for the REST API. Also the same key may be used in multiple places.