In CI: when installing tox, ignore existing versions
In CI: when installing tox, ignore existing versions

Tox 3.14.1 requires a newer virtualenv version than the one in
CentOS 7, so we need to add --ignore-installed to the pip command
line.

Change-Id: I784418971f2d4b02d5581ca8bd312ce1b3e395d5

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