Removing the default es hosts entry so that our autodetection works

[janheise]

currently, we have to explicitly unset (set empty) GRAYLOG_ELASTICSEARCH_HOSTS: "" in docker-compose.ymlor when starting docker containers to make autodetection work. this PR fixes it.

fixes https://github.com/Graylog2/graylog-docker/pull/258

Notes for Reviewers

• [ ] The commit history must be preserved - please use the rebase-merge or standard merge option instead of squash-merge
• [ ] Sync up with the author before merging