Image cache: move all of the variables to a common place

[OpenStack Infra]

https://review.openstack.org/59994
commit 6e6b4ad411cae3918cd2b54152c52fe3932dd723
Author: Gary Kotton <email address hidden>
Date: Wed Dec 4 05:36:21 2013 -0800

    Image cache: move all of the variables to a common place

    In addition to this it creates a base ImageCacheManager class.

    This moves common image cache config from the libvirt driver into
    a place that can be shared between drivers, nova.virt.imagecache
    The libvirt specific image caching variables are untouched.

    The other image cache fields that were in the nova.compute.manager
    have also been moved to the common place.

    These common configuration can be used by the
    blueprint multiple-image-cache-handlers and the
    blueprint vmware-image-cache-management.

    DocImpact
        The following configuration variables are now in the DEFAULT
        section:
            image_cache_manager_interval
            image_cache_subdirectory_name
            remove_unused_base_images
            remove_unused_original_minimum_age_seconds

    Change-Id: I92080f20c3d21ec348aff7d0a9795e773bcc2649

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/63475