def credential_file(file=nil)
      unless file.nil?
        dsl_options[:credential_file]=file
        dsl_options.merge!(Ec2.load_keys_from_credential_file(file))
      else
        fetch(:credential_file)
      end
    end
def credential_file(file=nil)
      unless file.nil?
        dsl_options[:credential_file]=file
        dsl_options.merge!(Ec2.load_keys_from_credential_file(file))
      else
        fetch(:credential_file)
      end
    end
Read credentials from credential_file if one exists