Add rabbitmq_exchange type and provider

This Pull Request adds `rabbitmq_exchange` type and provider using `rabbitmqadmin`. This is necessary to in order to setup Mcollective with the [rabbitmq connector](http://docs.puppetlabs.com/mcollective/reference/plugins/connector_rabbitmq.html).

The `rabbitmqadmin` command is not distributed with software packages, it must be downloaded from RabbitMQ itself after the `rabbitmq_management` plugin is activated, which opens the management port on which to download the script.