By Guy Faulconbridge and Catherine Belton LONDON (Reuters) -Two Russian billionaires, Mikhail Fridman and Oleg Deripaska, called for an end to the conflict triggered by President Vladimir Putin’s assault on Ukraine, with Fridman calling it a tragedy for both countries’ people. Billionaire Fridman, who was born in western Ukraine, told staff in a letter that the conflict was driving a wedge between the two eastern Slav peoples of Russia and Ukraine who have been brothers for centuries. “I was born in Western Ukraine and lived there until I was 17. My parents are Ukrainian citizens and live in L…