By Gabriela Baczynska BRUSSELS (Reuters) -The European Union’s executive body told its 27 member countries at a seminar on Monday that it was not possible to create a gas price cap that would not affect long-term contracts or supply security, two diplomatic sources told Reuters. After much wrangling at an all-night summit, EU leaders agreed last month to task the European Commission with proposing a temporary gas price cap in power generation and a temporary price corridor to bring down costs for consumers. But a compromise between those like France, Spain and Belgium that want a cap and the G…