major refactoring

* use dockerfiles/haproxy as base
* support etcd ambassador with polvi/simple-amb
* drop supervisord
* improve logging / error handling
* add HAPROXY_FORCE_HTTPS config variable (default: yes)
* restart haproxy only if config checksum changed
* detect ip/port order in etcd value
* declare possible ENV variables in Dockerfile
* renamed etcd/HAPROXY_BACKEND_SERVICE to env/HAPROXY_BACKEND
* ETCDCTL_PEER can be set via ENV too