Implement option to skip logging of QA commands

This patch adds an optional kwarg that allows skipping the "Command:
…" line in QA output. This is helpful if we only need to run some
maintenance actions, that are QA-related and not Ganeti-related.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>