By Pavel Polityuk and Conor Humphries KYIV/SLOVYANSK, Ukraine (Reuters) -Russian forces were conducting an all-out assault on Tuesday to encircle Ukrainian troops in twin cities straddling a river in eastern Ukraine, a battle which could determine the success or failure of Moscow’s main campaign in the east. Exactly three months after President Vladimir Putin ordered Russian forces into Ukraine, authorities in its second-largest city Kharkiv re-opened the underground metro, where thousands of civilians had sheltered for months under relentless bombardment. The move was evidence of Ukraine’s bi…