Generic throttling mechanism for pt-online-schema-change

pt-online-schema-change is a great tool. When used in a large replicated environment you often need to throttle it for various reasons, usually to prevent large replication lags. While there is an option to check slaves there may not be direct access to all slaves, or we may want to use a more sophisticated mechanism to determine if the changes need to be [temporarily] throttled. So rather than solve all possible issues, provide a mechanism to be able to call a generic script which can give this information. That script would be external to the tool and thus can be as flexible as needed.