Use VIRTUALENV_NO_DOWNLOAD for unupgrading pip
Use VIRTUALENV_NO_DOWNLOAD for unupgrading pip

tox -e py27 installs the latest setuptools-36.0.0 which is
broken due to missing dependency six module. We can disable
updating pip ugrade in order to fix the issue.

Change-Id: Icc712ec6b357c65fec96e9f85e193ab4e281aa06

openstack-packages/DLRN,openstack-packages/delorean,openstack-packages/DLRN,openstack-packages/delorean