Fix venv use in requirements propose_update.sh
Fix venv use in requirements propose_update.sh

This script creates a virtualenv to run a requirements script to update
constraints. virtualenv is not globally available anymore. Rather than
trying to call it directly, this now uses tox to set up the virtual
environment.

Change-Id: If00afb50c37b45aefbb45070da4efef3e43e62b2
Signed-off-by: Sean McGinnis <961eeb09b6d45516665e15b82e013282c4faf575@gmail.com>

openstack-infra/project-config,openstack-infra/project-config