grizzly: add support for compressing qcow2 snapshots

[Tom Fifield]

If https://review.openstack.org/#/c/20779/ is merged, a new feature is added to nova: support for compressing qcow2 snapshots

Adds a new configuration option:
libvirt_snapshot_compression=False (Default)

When set to True and the snapshot output format is qcow2, we pass
the -c option into qemu-img, which enables compression. This patch
also refactors the extract_image function to make construction of
the whole qemu-img command more dynamic.

 32 libvirt_opts = [
   33 cfg.BoolOpt('libvirt_snapshot_compression',
   34 default=False,
   35 help='Compress snapshot images when possible. This '
   36 'currently applies exclusively to qcow2 images'),
   37 ]

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/22174

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/22174
Committed: http://github.com/openstack/openstack-manuals/commit/ab66d6dfd5b57fe4fbdd3456a0ba9bd3fd43333b
Submitter: Jenkins
Branch: master

commit ab66d6dfd5b57fe4fbdd3456a0ba9bd3fd43333b
Author: Tom Fifield <email address hidden>
Date: Mon Feb 18 17:40:13 2013 +1100

    Doc libvirt_snapshot_compression option

    fixes bug 1111091

    libvirt_snapshot_compression -added in grizzly, enables compression
    for qcow2 options in libvirt. This patch adds the option to the table.

    Change-Id: Ie29ea1f8023c879bf1f6e2fc5bf77dcd1ec9fa61