tox.ini: Expose USER environment variable to execution environment
tox.ini: Expose USER environment variable to execution environment

In order to allow the use of the environment variable which informs
Ansible which user executed the playbook, we pass the USER env var
into the environment that tox builds.

Change-Id: Ia90ec0618d82088ed582ec81ec07a7c2b5458528
Implements: blueprint python-build-install-simplification
Signed-off-by: Jesse Pretorius <976bdec73968f87717846abb5bf1f2a1037e80c6@rackspace.co.uk>

openstack/openstack-ansible-apt_package_pinning,openstack/openstack-ansible-apt_package_pinning