Check ret for None in zuul_run_command

The check 'if not ret' not only matches a missing return code but also
the value 0 which is actually a successful run. Thus successful
commands end with an error 'Something went horribly wrong during task
execution'. This can be fixed by explicitly checking for None.

Also adds a successful shell task to the test_playbook test case which
fails without this patch.

Change-Id: If1d0721574a82e247659ab0f865ae6acfe12a6be
2 files changed