support reading systemctl config automatically on baremetal setups (#15066)

this allows for customers to use `mc admin service restart`
directly even when performing RPM, DEB upgrades. Upon such 'restart'
after upgrade MinIO will re-read the /etc/default/minio for any
newer environment variables.

As long as `MINIO_CONFIG_ENV_FILE=/etc/default/minio` is set, this
is honored.

cmd/common-main.go

@@ -526,9 +526,6 @@ func parsEnvEntry(envEntry string) (envKV, error) {
 func minioEnvironFromFile(envConfigFile string) ([]envKV, error) {
 	f, err := os.Open(envConfigFile)
 	if err != nil {
-		if os.IsNotExist(err) { // ignore if file doesn't exist.
-			return nil, nil
-		}
 		return nil, err
 	}
 	defer f.Close()
@@ -624,7 +621,7 @@ func loadEnvVarsFromFiles() {
 
 	if env.IsSet(config.EnvConfigEnvFile) {
 		ekvs, err := minioEnvironFromFile(env.Get(config.EnvConfigEnvFile, ""))
-		if err != nil {
+		if err != nil && !os.IsNotExist(err) {
 			logger.Fatal(err, "Unable to read the config environment file")
 		}
 		for _, ekv := range ekvs {