Add config option to enable or disable log executed SQL (#3726)

* add config option to enable or disable log executed SQL

* rename ShowSQL to LogSQL

models/models.go

@@ -270,7 +270,7 @@ func SetEngine() (err error) {
 	// WARNING: for serv command, MUST remove the output to os.stdout,
 	// so use log file to instead print to stdout.
 	x.SetLogger(log.XORMLogger)
-	x.ShowSQL(true)
+	x.ShowSQL(setting.LogSQL)
 	return nil
 }