* Lisbon agrees 4.9 bln BES bailout after weekend talks * Greek bonds suffer after Juncker plays down write-off (Adds fresh quotes, prices, Greek bond move) LONDON, Aug 4 (Reuters) - Portuguese bond yields fell on Monday after Lisbon agreed a near 5 billion euro ($6.6 billion) bailout of its biggest bank in a plan that reassured debt investors there would be no wider strain on public finances ...