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: I06e1528995924ca3c59405b04db28281a572cf6d
Implements: blueprint python-build-install-simplification
Signed-off-by: Jesse Pretorius <jesse.pretorius@rackspace.co.uk>

tox.ini

@@ -14,6 +14,7 @@ commands =
     /usr/bin/find . -type f -name "*.pyc" -delete
 passenv =
     HOME
+    USER
     http_proxy
     HTTP_PROXY
     https_proxy