The pipeline should return a `zaqar.common.pipeline.Pipeline` object instead of a list

[Flavio Percoco]

An example of the current code:

 108 @decorators.lazy_property(write=False)
 109 def queue_controller(self):
 110 stages = _get_storage_pipeline('queue', self.conf)
 111 stages.append(self._storage.queue_controller)
 112 return stages

It should actually return an instance of `pipeline.Pipeline` which is our proxy for these plugins.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/165153

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/165153
Committed: https://git.openstack.org/cgit/openstack/zaqar/commit/?id=286e11f081329becd8abb049deac17c2fd00ebe4
Submitter: Jenkins
Branch: master

commit 286e11f081329becd8abb049deac17c2fd00ebe4
Author: dynarro <email address hidden>
Date: Tue Mar 17 18:20:29 2015 +0100

    Make the Pipeline inmutable

    This patch makes pipeline inmutable by removing the append method.
    the pipeline metadata now returns a pipeline instance.

    Change-Id: I9b90933a83ca1d423d3718f438d3717c39ef78d4
    Closes-Bug: #1433158