By Andrew Osborn LONDON (Reuters) -In the twilight of the Soviet Union, Yevgeny Prigozhin was languishing in prison for theft. Now as the founder of Russia’s most powerful mercenary group, he is vying for Vladimir Putin’s favour by claiming a rare battlefield win in Ukraine. His aim is to leverage the success that Wagner Group, his mercenary outfit, had this week in pushing Ukrainian forces out of the salt mining town of Soledar, a move that revives Russian plans to seize more of eastern Ukraine after multiple defeats. Russia claimed victory on Friday, but Ukraine said on Saturday its troops w…