By Joseph Nasr and Mark Trevelyan BERLIN/LONDON (Reuters) -Russian President Vladimir Putin is demanding foreign buyers pay for Russian gas in roubles from Friday or else have their supplies cut, a move European capitals rejected and which Berlin said amounted to “blackmail”. Putin’s move, via a decree signed on Thursday, leaves Europe facing the prospect of losing more than a third of its gas supply. Germany, the most heavily reliant on Russia, has already activated an emergency plan that could lead to rationing in Europe’s biggest economy. Energy exports are Putin’s most powerful lever as he…