By Robin Emmott and Sabine Siebold BRUSSELS (Reuters) – NATO defence ministers are expected this week to set in motion a plan that could establish four multinational battlegroups in southeastern Europe in response to Russia’s military build-up in Ukraine, three diplomats said. Allied ministers meeting on Wednesday and Thursday will decide whether to order their military commanders to draw up plans to deploy battlegroups of some 1,000 troops each to Bulgaria and Romania, and possibly to Slovakia and Hungary. With NATO meeting as the United States warns of an imminent Russian invasion of Ukraine…