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/influxdb/influxdb.go

@@ -130,7 +130,7 @@ func (i *InfluxDB) Connect() error {
 
 			err = createDatabase(c, i.Database)
 			if err != nil {
-				log.Println("Database creation failed: " + err.Error())
+				log.Println("E! Database creation failed: " + err.Error())
 				continue
 			}
 
@@ -201,11 +201,11 @@ func (i *InfluxDB) Write(metrics []telegraf.Metric) error {
 	for _, n := range p {
 		if e := i.conns[n].Write(bp); e != nil {
 			// Log write failure
-			log.Printf("ERROR: %s", e)
+			log.Printf("E! InfluxDB Output Error: %s", e)
 			// If the database was not found, try to recreate it
 			if strings.Contains(e.Error(), "database not found") {
 				if errc := createDatabase(i.conns[n], i.Database); errc != nil {
-					log.Printf("ERROR: Database %s not found and failed to recreate\n",
+					log.Printf("E! Error: Database %s not found and failed to recreate\n",
 						i.Database)
 				}
 			}