Allow cinder-backup to use ceph

[OpenStack Infra]

https://review.openstack.org/220074
commit da75b0d1537a0c5588d685cbfd5917fb54bd8a88
Author: Matt Thompson <email address hidden>
Date: Wed Aug 5 15:31:46 2015 +0100

    Allow cinder-backup to use ceph

    This change updates ceph_client and os_cinder roles to allow
    cinder-backup to use ceph. We also create a new group called
    'cinder_backup' which allows us to only retreive the cinder backup key
    if cinder-backup is actually in use.

    To use, you would simply need to set cinder_service_backup_driver to
    cinder.backup.drivers.ceph in your user_variables.yml file.

    NOTE: You will need to update your
          /etc/openstack_deploy/env.d/cinder.yml in order for this change
          to execute successfully.

    Change-Id: Ib94effa40208bbc8de0f78c5487316be007adcf1
    Closes-Bug: #1481787
    Implements: blueprint ceph-block-devices
    DocImpact
    (cherry picked from commit 44d3f25de6daab8073137c9fc76edcdfc23c716f)