first commit with to do infer_service_tags

To do for now :

    Implement infer service tags function.
    Documentation

Except the "InferServiceTags", rest have been implemented and tested on stage with usage:

LoadPlugin write_opentsdb
   <Plugin write_opentsdb>
      <Node "example">
            Host "stage-opentsdb.ch.flipkart.com"
            Port "4242"
            Protocol "tcp"
            LogSendErrors true
            Prefix "collectd"
            EscapeChar "_"
            StoreRate false
            BufferSize 1428
            Tags "dc=ch service_name=opentsdb role=app"
            InferServiceTags false
      </Node>
   </Plugin>