Fix: step.ShellSequence doesn't catch stdout/stderr even if the paramete...

step.ShellSequence doesn't catch stdout/stderr even if the parameters want_stdout/want_stderr are set to True.

Note:
 - here's a example how to catch stderr/stdout with this patch applied
```
    factory.addStep(steps.ShellSequence(name="BuildSandBox",
                                        want_stdout=True,
                                        want_stderr=True,
                                        commands=[util.ShellArg(command=['rm', '-rf', 'sandbox'],
                                                                logfile="rm",),
                                                  util.ShellArg(command=['make', 'buildsandbox',  'args="sandbox"'],
                                                                logfile="sandbox",),]))
```
actually, in the implementation, those args are completly ignored!