Mitigate viper bug in failing to bind env to nested config

[haoming29]

Ref: spf13/viper#761

As Fabio reported, when setting the Server.TLSCACertificateDirectory config via environment variable, i.e. PELICAN_SERVER_TLSCACERTIFICATEDIRECTORY=/etc/grid-security/certificates, the value isn't propagated to xrootd config. This is because we used viper.Unmarshal to unmarshal viper configuration to our defined xrootd configuration struct:

pelican/xrootd/xrootd_config.go

Lines 680 to 682 in b269e79

	if err := viper.Unmarshal(&xrdConfig, viper.DecodeHook(combinedDecodeHookFunc())); err != nil {
	return "", errors.Wrap(err, "failed to unmarshal xrootd config")
	}

However, given the bug in viper, the values of nested fields are lost in unmarshaling. People in the the referred issue proposed a couple of work-around and we should pick one and replace all our instances where we used viper.Unmarshal.