Copied the 2.0.0 release.
More configurable RabbitMQ script generation.

- RabbitMQ directory locations are now configurable using the
  `mnesia-base`, `log-base`, and `pidfile` options.

- RabbitMQ plugins can now be enabled by creating a file named
  `etc/rabbitmq_enabled_plugins`.

- The `location` variable is now exposed so other recipes can depend on it.
Made compatible with RabbitMQ 3.1.1