Pass OS_*_CAPTURE env variables

The unit tests look for the OS_STDOUT_CAPTURE, OS_STDERR_CAPTURE,
and OS_LOG_CAPTURE environment variable values, but these are not
being passed down through tox. We have to explicitly allow them with
the use of the passenv tox.ini variable.

Change-Id: Ib33cb3b024ac9d61b590771071050177370956d1
diff --git a/tox.ini b/tox.ini
index 79ea939..ad0aa31 100644
--- a/tox.ini
+++ b/tox.ini
@@ -9,7 +9,7 @@
          STATSD_PORT=8125
          VIRTUAL_ENV={envdir}
          OS_TEST_TIMEOUT=30
-passenv = ZUUL_TEST_ROOT
+passenv = ZUUL_TEST_ROOT OS_STDOUT_CAPTURE OS_STDERR_CAPTURE OS_LOG_CAPTURE
 usedevelop = True
 install_command = pip install {opts} {packages}
 deps = -r{toxinidir}/requirements.txt