reserve riemann keyword: host, ttl, state, description

opentsdb can define tags with format  tagk1=tagv1 ,  tagk2=tagv2  ... , but riemann only accept tags = [tagv1, tagv2,...], so version 0.0.1 , this plugin will take all  tag value into  [tagv1, tagv2,...] event the  riemann keyword :host, ttl, state, description. That would be frustated  if you need to utilize riemann's state keyword, so I  make a pull request.  I've already put it in the production environment.