By Michelle Nichols UNITED NATIONS (Reuters) – U.S. Ambassador to the United Nations, Linda Thomas-Greenfield, canceled a planned visit to Liberia this week to focus instead on U.N. diplomacy on Russia’s troop build-up near Ukraine as Washington warns Moscow could invade at any time. Tensions over the Ukraine crisis spilled into the U.N. Security Council two weeks ago at a public meeting requested by Washington. Russia, which denies planning an invasion, made a failed bid to stop the council discussion. While all members were able to air their views openly, there was no action by the 15-member…