By Gwladys Fouche and Terje Solsvik OSLO (Reuters) -A planned strike among Norwegian oil and gas workers next week could cut the country’s overall petroleum output by around 8% unless a last-minute agreement is found over wage demands, a Reuters calculation showed on Friday. The conflict over wages between oil companies and offshore platform employees could slash gas production by more than 10% at a time of high prices and tight supply in Europe. Norway’s Lederne union, which on Thursday announced that its members had voted down a wage deal and would start a strike on July 5, said on Friday th…