MOSCOW/KIEV, Jan 5 (Reuters) - Europe may have breathed a collective sigh of relief after Russia and Ukraine signed a long-term gas supply deal, averting a possible repetition of the New Year supply cutbacks that unnerved the continent. But after a closer look at the complex pact signed on Wednesday, outsiders may yet have cause for concern about the region's energy security, especially after Ukraine's ex-premier Yulia Tymoshenko launched a legal challenge to stop the deal. The accord uses as middleman a little-known Swiss-based joint venture called RosUkrEnergo, owned half by Russia's gas monopoly Gazprom (GAZPPE.RTS: Quote, Profile, Research) and...