Pass ZUUL_TEST_ROOT through tox

This is so that one might, for example, run the zuul tests out of
a tmpfs.

Change-Id: I8e1d5b11c83e5f782f92d95b0d09632312bf310d

tox.ini

@@ -9,6 +9,7 @@ setenv = STATSD_HOST=127.0.0.1
          STATSD_PORT=8125
          VIRTUAL_ENV={envdir}
          OS_TEST_TIMEOUT=30
+passenv = ZUUL_TEST_ROOT
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt