Feb 27 (Reuters) - Private equity firm Blackstone Group LP said on Monday it has agreed to invest $2 billion in Cheniere Energy Partners LP, in a deal that would help the company finance construction of a gas-liquefaction plant for export markets. Houston-based LNG developer Cheniere will use the investment to build its first export plant in Sabine Pass, Louisiana. Blackstone's investment was made via special units in Cheniere that will convert into common units once the facility begi