Add `_max_messages_post_size` and `_default_message_ttl` for queue

[OpenStack Infra]

https://review.openstack.org/265723
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/zaqar" is set up so that we directly report the documentation bugs against it. If this needs changing, the docimpact-group option needs to be added for the project. You can ask the OpenStack infra team (#openstack-infra on freenode) for help if you need to.

commit 7c6d52ccbf59989f75ed1e0ad90807765c505618
Author: Fei Long Wang <email address hidden>
Date: Mon Jan 11 15:32:19 2016 +1300

    Add `_max_messages_post_size` and `_default_message_ttl` for queue

    Now Zaqar supports metadata as the attributes of a queue, so that user
    can fill it with some key-value pair. However, Zaqar itself isn't
    using it very much(only the '_flavor'). Meanwhile, we have some
    configurations defined in zaqar.conf against messages or claims which
    are global defined. For example, the max message size. So the idea is
    adding more reserved attributes for queue so that the queues of Zaqar
    are more flexible. This patch adds the `_max_messages_post_size` and
    `_default_message_ttl` for message posting, tests are added as well.

    APIImpact
    DocImpact

    blueprint more-reserved-queue-attributes

    Change-Id: I7ba84b6e862ff15684a1b1d919c0fed60f4c58b2