update pipeline to call teardown of output workers when :workers > 1

fixes https://github.com/elasticsearch/logstash/issues/2178
Why not move the teardown of @worker_plugins into the  [LogStash::Outputs::Base#teardown](https://github.com/elasticsearch/logstash/blob/master/lib/logstash/outputs/base.rb).

This way, pipeline keeps its `@outputs.each(&:teardown)` and it's teardown's responsibility to destroy each plugin's workers.