[vs]: Add pip install six (#1978)

sonic-utilities requires six

Signed-off-by: Shu0T1an ChenG <shuche@microsoft.com>

platform/vs/docker-sonic-vs/Dockerfile.j2

@@ -46,6 +46,7 @@ RUN apt-get install -y net-tools \
 
 RUN pip install setuptools
 RUN pip install py2_ipaddress
+RUN pip install six
 
 {% if docker_sonic_vs_debs.strip() -%}
 # Copy locally-built Debian package dependencies