show default attributes for queue

Currently Zaqar will support more built-in/reserved attributes in queue (https://blueprints.launchpad.net/zaqar/+spec/more-reserved-queue-attributes).
We add two important attributes MaximumMessageSize and MessageRetentionPeriod.

So it's very useful for users that if they create a queue without metadata, when they query queue, Zaqar add those two attributes from config to queue metadata and then return. That will not be stuck if config is changed and user also can know what value it is.