libvirt: deprecated volume_drivers config parameter

In Icehouse the 'vif_driver' parameter was deprecated

  commit 9f6070e194504cc2ca2b7f2a2aabbf91c6b81897
  Author: Lars Kellogg-Stedman <lars@redhat.com>
  Date:   Fri Apr 4 14:58:12 2014 -0400

    mark vif_driver as deprecated and log warning

and then in Juno removed

  commit 7561c8ded211d53e8745d1420a73b82bd0fc35cf
  Author: Daniel P. Berrange <berrange@redhat.com>
  Date:   Thu Jul 24 11:53:47 2014 +0100

    libvirt: remove 'vif_driver' config parameter

The 'volumes_drivers' parameter does the same job as the
'vif_driver' parameter, but for volumes instead of vifs.
The reasons for deprecating it, in preparation for its
removal in Lxxxx are thus the same.

The API for the LibvirtBaseVolumeDriver class is not
considered to be a public extension point. As such it
is not required or expected to remain compatible across
releases.

The main reason for its existance is for people who are
developing new cinder drivers, to be able to write out
of tree nova volume drivers. The recommended practice
is to instead work in-tree to nova on a branch. This is
an approach that would be required in order to submit
the work for review and eventual merge in any case.

DocImpact: libvirt.volume_drivers config param for
           nova.conf is deprecated, to be removed in
           the Lxxxx release
Related-bug: #1362191
Change-Id: I1f884f36e24fe478922671ca3eaf6b7c21fd8a5a
Merge "libvirt: deprecated volume_drivers config parameter"