Add rabbitmq_ssl parameter to enable SSL transport to RabbitMQ

This new parameter enables SSL transport when connecting to RabbitMQ.  This way
you can use SSL to connect to RabbitMQ without configuring it for certificate
authentication (rabbitmq_ssl_private_key and rabbitmq_ssl_cert_chain parameters.)

If you have rabbitmq_ssl => true with no other rabbitmq_ssl parameters, the
rabbitmq.json config file will have an empty ssl { } stanza in it.  This makes
Sensu use SSL transport when connecting to RabbitMQ, but it will not attempt
certification authentication.