Support default config paths

precedence will be:

 1. --config command-line option
 2. $TELEGRAF_CONFIG_PATH
 3. $HOME/.telegraf/telegraf.conf
 4. /etc/telegraf/telegraf.conf

internal/config/config.go

@@ -412,8 +412,35 @@ func (c *Config) LoadDirectory(path string) error {
 	return nil
 }
 
+// Try to find a default config file at these locations (in order):
+//   1. $TELEGRAF_CONFIG_PATH
+//   2. $HOME/.telegraf/telegraf.conf
+//   3. /etc/telegraf/telegraf.conf
+//
+func getDefaultConfigPath() (string, error) {
+	envfile := os.Getenv("TELEGRAF_CONFIG_PATH")
+	homefile := os.ExpandEnv("${HOME}/.telegraf/telegraf.conf")
+	etcfile := "/etc/telegraf/telegraf.conf"
+	for _, path := range []string{envfile, homefile, etcfile} {
+		if _, err := os.Stat(path); err == nil {
+			log.Printf("Using config file: %s", path)
+			return path, nil
+		}
+	}
+
+	// if we got here, we didn't find a file in a default location
+	return "", fmt.Errorf("No config file specified, and could not find one"+
+		" in $TELEGRAF_CONFIG_PATH, %s, or %s", homefile, etcfile)
+}
+
 // LoadConfig loads the given config file and applies it to c
 func (c *Config) LoadConfig(path string) error {
+	var err error
+	if path == "" {
+		if path, err = getDefaultConfigPath(); err != nil {
+			return err
+		}
+	}
 	tbl, err := parseFile(path)
 	if err != nil {
 		return fmt.Errorf("Error parsing %s, %s", path, err)