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/inputs/ceph/ceph.go

@@ -100,12 +100,12 @@ func (c *Ceph) gatherAdminSocketStats(acc telegraf.Accumulator) error {
 	for _, s := range sockets {
 		dump, err := perfDump(c.CephBinary, s)
 		if err != nil {
-			log.Printf("error reading from socket '%s': %v", s.socket, err)
+			log.Printf("E! error reading from socket '%s': %v", s.socket, err)
 			continue
 		}
 		data, err := parseDump(dump)
 		if err != nil {
-			log.Printf("error parsing dump from socket '%s': %v", s.socket, err)
+			log.Printf("E! error parsing dump from socket '%s': %v", s.socket, err)
 			continue
 		}
 		for tag, metrics := range *data {
@@ -293,7 +293,7 @@ func flatten(data interface{}) []*metric {
 			}
 		}
 	default:
-		log.Printf("Ignoring unexpected type '%T' for value %v", val, val)
+		log.Printf("I! Ignoring unexpected type '%T' for value %v", val, val)
 	}
 
 	return metrics