LOGSTASH-689: prefix OpenTSDB message with 'put '

Fixes sending metrics to OpenTSDB, as per the bug description. Have tested, and it works with the following configuration:
```
  opentsdb { tags => [ "metric" ] debug => true host => "localhost" port => 4242 
			 metrics => ["log.messages", "%{INFO.count}", "severity","info" ]
		   } 
  opentsdb { tags => [ "metric" ] debug => true host => "localhost" port => 4242 
			 metrics => ["log.messages", "%{DEBUG.count}", "severity","debug" ]
		   } 
```
Sample messages sent to OpenTSDB:
```
put log.messages 1358206603 37 severity=info
put log.messages 1358206603 446 severity=debug
```