Handle localrc early enough in stackrc
Handle localrc early enough in stackrc

We need to source the environment overrides before they get evaluated.
Otherwise e.g. USE_PYTHON3 is factually being ignored for some settings.

Also fix creating python3 venvs by using the "virtualenv" command for
that task.

Change-Id: I16c78a7fef80372d9a1684c3256c5b50b052ecae

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