Netvirt IT uses DockerOvs() in ovsdb.it-utils. DockerOvs uses docker-compose.

In Jenkins, docker-compose is installed in a virtual environment. So, when running DockerOvs on jenkins, we need to switch to a virtual environment (venv) to get access to docker-compose.

New flag, -DuseVenv=true, will trigger DockerOvs to make system call to switch to virtual environment.