Make it possible to disable tty in qa_utils.GetCommandOutput

The tty can have strange side effect as it seems to replace \n with
\r\n. In case of simply catting a file and compare it's output this
will break. This patch adds the possibility to disable tty if needed.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>