HTTP API listener: make it possible to disable non-TLS listeners

[michaelklishin]

Plain (non-TLS) AMQP 0-9-1 and AMQP 1.0 listeners can be disabled. The same should be possible for the non-TLS HTTP API listener.

[illotum]

I meant to quickly knock it off, but discovered it can be done already.

If the ssl listener is configered and no management.tcp settings are in the config, HTTP socket will be disabled. Can be verified by enabling the plugin with this minimal config (and certs present):

management.ssl.port = 8443
management.ssl.certfile = /etc/rabbitmq/example.com.crt
management.ssl.keyfile = /etc/rabbitmq/example.com.key

# management.tcp.port = 8080

See rabbit_mqmq_app:get_listeners_config/0.