Use https to download rabbitmqadmin tool when $rabbitmq::ssl => true

If $rabbitmq::admin_enable is set, the rabbitmqadmin tool is downloaded
via a http URL on the RabbitMQ management service.  However, this fails
when $rabbitmq::ssl is set, because in that case the managment service is
configured for https only (there is no http listener.)
Proper bool logic
Merge pull request #234 from misterdorm/rmqadminssl

Use https to download rabbitmqadmin tool when $rabbitmq::ssl => true