Fix virtualenv

virtualenv $WORKSPACE/venv created a python3.4 environment
I have installed python36u on the builer and symlinked
/usr/bin/python3 to /usr/bin/python36

I have tested this change and it worked for the broken
job:
https://build.opnfv.org/ci/job/releng-release-hunter-merge/

Change-Id: Ibdc3218c47b5103f28a8b6f730962434972b76be
Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>