[rabbitmq] no way to turn off ssl_verify in RabbitMQ instance

[iancward]

I added support for the ssl_verify RabbitMQ integration in #383; however, now that we're actually trying to use it, there's no way to actually to disable ssl verification because the template or's the attribute value with true: https://github.com/DataDog/chef-datadog/blob/master/templates/default/rabbitmq.yaml.erb#L11.

I think what I was actually trying to do there was set it to true if the variable was undefined, and inadvertently prevented users from being able to turn it off.