Support the use of the $ character in the config file

[clinmanc]

As shown below, if the password contains the $ character, the read password is uhQhu4watyTgn$, resulting in failure to connect to the redis

[[resource]]
name = "alertmanager"

  [resource.backend]
    [resource.backend.redis]
      nodes    = ["127.0.0.1:6379"]
      password = "uhQhu4watyTgn$Q$"
      prefix   = "/alert"
      interval = 5
      keys     = ["/*"]

The code that causes this problem

func readFileAndExpandEnv(path string) ([]byte, error) {
	buf, err := ioutil.ReadFile(path)
	if err != nil {
		return buf, errors.Wrap(err, "read file failed")
	}
	// expand the environment variables
	buf = []byte(os.ExpandEnv(string(buf)))
	return buf, nil
}

It can be fixed in the following way

https://play.golang.org/p/JOJ7WLIUY3