Capture logs when running unit tests

The unit test logs are mixed up in the output of subunit-trace.
Adding OS_LOG_CAPTURE=1 by default to avoid that.

Change-Id: Ie322795c8638963b276c4bd54b845331c3567370
diff --git a/tox.ini b/tox.ini
index 6f37d00..c4f51c8 100644
--- a/tox.ini
+++ b/tox.ini
@@ -17,6 +17,7 @@
 setenv =
     VIRTUAL_ENV={envdir}
     OS_TEST_PATH=./tempest/tests
+    OS_LOG_CAPTURE=1
     PYTHONWARNINGS=default::DeprecationWarning
     BRANCH_NAME=master
     CLIENT_NAME=tempest