Avoid re-configuring (and validating) SSL/TLS configuration on `CONFIG
SET` when TLS is not actively enabled for incoming connections, cluster
bus or replication.
This fixes failures when tests run without `--tls` on binaries that were
built with TLS support.
An additional benefit is that it's now possible to perform a multi-step
configuration process while TLS is disabled. The new configuration will
be verified and applied only when TLS is effectively enabled.
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