Community plugins

This pull requests adds a new recipe, rabbitmq::community_plugins, that downloads, installs and enables community plugins.

To specify a plugin, set the attribute `node['rabbitmq']['community_plugins']['PLUGIN_NAME']` to `'DOWNLOAD_URL'`. For example, to use the RabbitMQ priority queue plugin (https://github.com/rabbitmq/rabbitmq-priority-queue), set the attribute `node['rabbitmq']['community_plugins']['rabbitmq_priority_queue']` to `'https://www.rabbitmq.com/community-plugins/v3.4.x/rabbitmq_priority_queue-3.4.x-3431dc1e.ez'`.