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
1 file changed