Fix libvirt-python install while stacking

Fix libvirt-python install while stacking

Problem:
1. Devstack force installs libvirt-python from
   pip repo, during which the compatibility with the
   installed version of libvirt package is validated.
2. Queens was using libvirt-python (3.10)
3. Recently libvirt version got upgraded in CentOS repo
   ref: http://mirror.centos.org/centos-7/7.6.1810/os/x86_64/Packages/
   causing the compatibility with libvirt-python to fail.

Fix:
 Force devstack to use the libvirt-python latest version
 that is compatiable with the libvirt in CentOS repo.

Change-Id: I41d6d4529d249688947af14efa668afb7a29cd95
Signed-off-by: gvrangan <venkatrangang@hcl.com>