Allow for multiple store backends for Glance

[OpenStack Infra]

https://review.openstack.org/265832
Dear bug triager. This bug was created since a commit was marked with DOCIMPACT.
Your project "openstack/openstack-ansible" 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 8341a62350c9139d57e970643fc892cc3509bbb7
Author: Bjoern Teipel <email address hidden>
Date: Wed Dec 9 17:28:24 2015 -0600

    Allow for multiple store backends for Glance

    This implementation will add a new `user_variables.yml` override
    glance_additional_stores which is set with the default values http, cinder
    The operator can now independently enable Glance store protocols besides
    glance_default_store.

    A example configuration can be

    glance_default_store = rbd
    glance_additional_stores:
      - swift
      - http
      - cinder

    to maintain glance images over RBD and Swift/HTTP.
    This is particular useful when adding new storage protocols
    to glance while retaining access to the existing protocols/images.

    DocImpact
    Closes-Bug: #1524544

    Change-Id: I2517fd5170bc67460fe3f66e0a0215af390343bc
    (cherry picked from commit a4a29ecf88b323d4c798af7eb52424a5cbed1417)