rapid-cdn: Implement expert SSL downgrade
expert-backend-allow-downgrade-ssl allows to configure each node to downgrade SSL negotiation with the backends to insecure, OpenSSL 1 style, ciphers and algorithms. It's implemented only per node as it shall be only used in some specific circumstances and the cluster default (by default) is false. Such separation allows to setup each node independently, as it requires node restart. backend-haproxy is hashing backend-haproxy-wrapper:output in order to be reactive on applied change to the configuration; this will result with node restart after the configuration.