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.