Updated plugin interface to work in Docker environment by default.
* Plugins assume `rabbitmq` as name of RabbitMQ broker instead of `localhost`.
* Plugins accept a number of Waggle environment variables:
** WAGGLE_PLUGIN_ID
** WAGGLE_PLUGIN_VERSION
** WAGGLE_PLUGIN_INSTANCE
** WAGGLE_PLUGIN_HOST
** WAGGLE_PLUGIN_PORT
** WAGGLE_PLUGIN_USERNAME
** WAGGLE_PLUGIN_PASSWORD
** WAGGLE_NODE_ID
** WAGGLE_SUB_ID