By Fiona Ortiz and Elisabeth O'Leary MADRID (Reuters) - Spain's rising political star is a 61-year-old former Socialist whose message of changing the system from within is drawing voters in despair at economic ruin and official corruption in the euro zone's fourth biggest economy. Lacking the raucous anti-establishment appeal of Italy's Beppe Grillo and Greek leftist hero Alexis Tsipras, Rosa Diez relies on sharp debate to deliver her reform message to a country pushed to the brink by the euro zone debt crisis. ...