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: Ib6690d5a1cda3ca6d56752f910c24954e5711587
Implements: blueprint python-build-install-simplification
Signed-off-by: Jesse Pretorius <976bdec73968f87717846abb5bf1f2a1037e80c6@rackspace.co.uk>

openstack/openstack-ansible-openstack_hosts,openstack/openstack-ansible-openstack_hosts