Currently we're installing pip packages into the venv
unconstrained, resulting in the possibility of an
inconsistent build. This has luckily been covered by
the repo build process only making a single, correct
version of a python package available. However, for
role tests it is now failing because it's pulling in
a new version of libvirt-python which doesn't work
with the libvirt version used in ocata.
Change-Id: I1607685ac6b30b698dacf5f05c4f5639ae53df44