Using venv, which is part of python3, we avoid an extra dependency on
the virtualenv package. For Debuntu, which splits this out into a
separate package, add this to debs/general.
This is part of the infra efforts to ship "plain" nodes without any
dependencies installed. While devstack can re-install virtualenv, we
don't need any features it provides and it means one less dependency.
Change-Id: I3c323640f288e57581a4eb8adba2a08d0b0cbd8f
e6852eadb1