def servers(*roles)
      if roles.empty?
        @servers
      else
        roles.map do |role|
          @servers.select { |server| server.roles.include?(role) }
        end.flatten.uniq
      end
    end
def servers(*roles)
      if roles.empty?
        @servers
      else
        roles.map do |role|
          @servers.select { |server| server.roles.include?(role) }
        end.flatten.uniq
      end
    end
get list of server objects (optionally matching list of roles)