Create virtualenv with abstracted VIRTUALENV_CMD
Create virtualenv with abstracted VIRTUALENV_CMD

Just calling "virtualenv" makes a Python 2 based environment;
setuptools just dropped Python 2 support (as Python 2 reached EOL in
Jan 2020) so this has now become a breakage.

Although the Python 2 path won't work, use the abstracted command.
This should stop us having to revisit this for any future cleanups (or
switing to venv, etc).

Change-Id: I531e971b78491a9276753c0d86b04c4adbd224aa

openstack-dev/devstack,nawawi/openstack,openstack-dev/devstack,nawawi/openstack