LONDON, Nov 13 (Reuters) - Russia's Yevgeny Prigozhin, a close ally of President Vladimir Putin, said on Sunday that a former mercenary who was filmed being executed by a sledgehammer blow to the head after changing sides in the Ukraine war was a traitor. Prigozhin, a Russian businessman who founded the Wagner private military group, was responding to an unverified video distributed on Telegram that showed a man identified as a former Wagner mercenary being executed after admitting that he had changed sides in September to "fight against the Russians".