ElasticSearch output plugin to support multiple hosts and enhance stability 

ElasticSearch output plugin to support multiple hosts and enhance stability
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1. Make elasticSearch output plugin to support mutiple hosts and round robin balance, which will help a lot to balance the resource and performance of whole elasticsearch cluster.
2. Improve stability by handling exception and retry with left servers while flushing bulk, which will reduce the possibility of data loss largely.
The template part only needs to be done towards a single host. there is no use to do that on all hosts since ES takes care of that