NEW YORK, Aug 10 (Reuters) - Two of Canada's biggest pension funds are exploring options including a sale of Cubico Sustainable Investments that could value the renewable energy firm at about $6 billion or more, including debt, according to people familiar with the matter.Montreal-based Public Sector Pension (PSP) Investment Board and the Ontario Teachers' Pension Plan (OTPP) are looking to appoint a financial advisor in the coming weeks, the sources said, adding the sale could take several months to complete.The sources, who requested anonymity as the matter is confidential, cautioned...