Major Logging Overhaul

in this commit:

- centralize logging output handler.
- set global Info/Debug/Error log levels based on config file or flags.
- remove per-plugin debug arg handling.
- add a I!, D!, or E! to every log message.
- add configuration option to specify where to send logs.

closes #1786

plugins/outputs/opentsdb/opentsdb_http.go

@@ -164,9 +164,11 @@ func (o *openTSDBHttp) flush() error {
 
 	if resp.StatusCode/100 != 2 {
 		if resp.StatusCode/100 == 4 {
-			log.Printf("WARNING: Received %d status code. Dropping metrics to avoid overflowing buffer.", resp.StatusCode)
+			log.Printf("E! Received %d status code. Dropping metrics to avoid overflowing buffer.",
+				resp.StatusCode)
 		} else {
-			return fmt.Errorf("Error when sending metrics.Received status %d", resp.StatusCode)
+			return fmt.Errorf("Error when sending metrics. Received status %d",
+				resp.StatusCode)
 		}
 	}