functests: use ensure-virtualenv
functests: use ensure-virtualenv

This ensures the virtualenv tool is available on all testing nodes.

Depends-On: https://review.opendev.org/723309
Change-Id: Ic52a380dab9faa0f65d3c1ebe0bff6f69736e3cd

openstack/diskimage-builder,openstack/diskimage-builder,switch-ch/diskimage-builder,switch-ch/diskimage-builder