Make queues lazy in subscriptions

[OpenStack Infra]

https://review.openstack.org/320749
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 4c2b7e04dbca0ae1e5d3480c8bad60dcbfac8ff8
Author: Eva Balycheva <email address hidden>
Date: Wed May 25 05:19:25 2016 +0300

    Make queues lazy in subscriptions

    Currently queues are lazy on operations with messages, but not with
    subscriptions. It means the user is forced to pre-create a queue before
    creating a subscription to the queue, which is not very convenient.

    Also currently if a queue has a subscription, the subscription will
    stay on even if the user will delete the queue. But even if so Zaqar
    will still work normally. This is strange that subscriptions can exist
    with deleted corresponding queues, but it's impossible to create a
    subscription to yet unexising queue.

    This patch makes queues lazy also on operations with subscriptions, so
    the user will be able to subscribe to yet unexisting queue.

    Also this patch modifies tests, to make sure Zaqar's subscriptions
    work both with pre-created queues and non-existing queues.

    APIImpact
    DocImpact
    Implements: blueprint lazy-queues-in-subscriptions

    Change-Id: I814b503243c4e06e74acc6b709bda4269df889e9