Add flag '--dry' allowing test run
Add flag '--dry' allowing test run

When enabled HAProxy config is only printed to stdout, without
applying any changes.

mesosphere/marathon-lb,JamesAwesome/marathon-lb,matt-deboer/marathon-lb,knesenko/marathon-lb,GrovoLearning/marathon-lb,vixns/marathon-lb,knesenko/marathon-lb,matt-deboer/marathon-lb,GrovoLearning/marathon-lb,mesosphere/marathon-lb,matt-deboer/marathon-lb,JamesAwesome/marathon-lb,vixns/marathon-lb,JamesAwesome/marathon-lb,GrovoLearning/marathon-lb,mesosphere/marathon-lb,knesenko/marathon-lb,vixns/marathon-lb