role_list_from (Capistrano::Configuration::Servers)

    # File lib/capistrano/configuration/servers.rb, line 83
          def role_list_from(roles)
            roles = roles.split(/,/) if String === roles
            roles = build_list(roles)
            roles.map do |role|
              role = String === role ? role.strip.to_sym : role
              raise ArgumentError, "unknown role `#{role}'" unless self.roles.key?(role)
              role
            end
          end