Add passenv option to tox.ini

Tox 2.0 introduced env variable separation, meaning that env vars set
in the env calling tox will not automatically be in the test env. This
breaks some assumptions we have in tempest. This commit adds all the
env vars which are used by testr or tempest directly for running tests
in the hope of making things work on tox 2.0 and older versions.

Change-Id: Icfe4bb5df0e48cbc9fcdbc602656672295d1887e
diff --git a/tox.ini b/tox.ini
index 88d1302..b44d715 100644
--- a/tox.ini
+++ b/tox.ini
@@ -13,6 +13,13 @@
 [testenv]
 setenv = VIRTUAL_ENV={envdir}
          OS_TEST_PATH=./tempest/tests
+passenv = OS_STDOUT_CAPTURE
+          OS_STDERR_CAPTURE
+          OS_TEST_TIMEOUT
+          OS_TEST_LOCK_PATH
+          OS_TEST_PATH
+          TEMPEST_CONFIG
+          TEMPEST_CONFIG_DIR
 usedevelop = True
 install_command = pip install -U {opts} {packages}
 whitelist_externals = *