* Equities move lower in U.S., Europe* Dollar drops to 6-1/2 month low versus currency basket* Tension continues between Russia, Ukraine(Updates to open of U.S. trading)By Ryan VlastelicaNEW YORK, May 6 (Reuters) - Stock markets around the world fell on Tuesday as investors found few reasons to keep pushing shares higher, with many indexes near record levels and ongoing geopolitical uncertainty.Th