Add a config option to enable "default" pools

[Flavio Percoco]

We recently changed the pools behavior to allow for default pools to be created. That is, when a pool is not found, it will register a pool with id `None` and use that instead. Zaqar does this using the available configurations for `message_storage`.

Unfortunately, this breaks backwards compatibility as it changes the behavior of the API. For instance, a queue creation won't fail if the aforementioned requirements are met. In order to preserve backwards compatibility, we should make this optional and default to the old behavior. Eventually, the new behavior should become the default one.

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/189374
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=bfcc60c170369b3a3e3ab116e477d7a6e706e1be
Submitter: Jenkins
Branch: master

commit bfcc60c170369b3a3e3ab116e477d7a6e706e1be
Author: Flavio Percoco <email address hidden>
Date: Mon Jun 8 19:22:10 2015 +0200

    Try to use a default pool when creating a queue

    We recently enabled default pools creation, which will use the default
    `message_storage` if present. This patch extends that to queue's
    creation where we try to register a queue in the catalogue but we fail
    if no pools were found.

    With this patch, the failure will happen after checking if there's a
    default pool that can be used.

    In addition to the above, this commit adds a new option
    `enable_virtual_pool` that makes default pools registration optional.
    This is to avoid breaking backwards compatibility on the deployments
    already running Zaqar.

    DocImpact

    Closes-bug: #1462008
    Closes-bug: #1463110
    Change-Id: Ieb8d2671066fc909f422bd74fd20070bc0f9cf11