libvirt IPv6 network support

This commit makes two modifications geared towards supporting IPv6
network configuration set in PDF/IDF:

  * libvirt-net.xml.j2: update template for libvirt network to correctly
 support IPv6 addresses defined in PDF/IDF.
  * libvirt-vm.xml.j2: implement dynamic allocation of PCI slots in
  order to support additional networks defined in PDF/IDF.

An additional change pins the version of Ansible to 2.9.6, in line with
that used by infra/engine and other friends.

Signed-off-by: Cian Johnston <cian.johnston@est.tech>
Change-Id: I35490a821d3e091eb9eb63a14c1f04cc3d5ed7c4
diff --git a/test-requirements.txt b/test-requirements.txt
index 13213c7..b301526 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -5,6 +5,7 @@
 
 # NOTE: engine test requirements
 tox==3.14.3
+ansible==2.9.6
 ansible-lint==4.1.0
 yamllint==1.19.0
 shellcheck-py==0.7.0.1