Bring stdout/stderr capturing in line w/ nova

In .testr.conf, we reference OS_STDOUT_CAPTURE / OS_STDERR_CAPTURE
while in novaclient/tests/utils.py we reference OS_STDOUT_NOCAPTURE
and OS_STDERR_NOCAPTURE.  This change brings things more in line
with nova project by referencing OS_STDOUT_CAPTURE / OS_STDERR_CAPTURE
in both locations.

Change-Id: I22efdec84bef78e99d1d95303cadade6011d76a2
Fixes: bug #1192997
Merge "Bring stdout/stderr capturing in line w/ nova"