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: Id0be68dff2a6f12b06b514fa4287f095ccb6fd9d
Implements: blueprint python-build-install-simplification
Signed-off-by: Jesse Pretorius <976bdec73968f87717846abb5bf1f2a1037e80c6@rackspace.co.uk>

openstack/openstack-ansible-pip_install,openstack/openstack-ansible-pip_install