remove runtime dep on python-pbr

Change-Id: I6f247f64c561d49c13e3c2f67401fe150b6d47b6
libvirt: Fix root disk leak in live mig

This patch makes sure that i_create_images_and_backing method of the
libvirt driver (called in several places, but most problematic one is
the call in the pre_live_migration method) creates all the files the
instance needs that are not present.

Prioir to this patch - the method would only attempt to download the
image, and if it did so with the path of the ephemeral drives, it could
expose the image to other users as an ephemeral devices. See the related
bug for more detaiis.

After this patch - we properly distinguish between image, ephemeral and
swap files, and make sure that the imagebackend does the correct thing.

Closes-bug: #1251590

Co-authored-by: Loganathan Parthipan <parthipan@hp.com>

Change-Id: I78aa2f4243899db4f4941e77014a7e18e27fc63e