Quick support to server level WORM (#5602)

This is a trival fix to support server level WORM.  The feature comes
with an environment variable `MINIO_WORM`.

Usage:
```
$ export MINIO_WORM=on
$ minio server endpoint
```

cmd/common-main.go

@@ -158,4 +158,7 @@ func handleCommonEnvVars() {
 			globalIsStorageClass = true
 		}
 	}
+
+	// Get WORM environment variable.
+	globalWORMEnabled = strings.EqualFold(os.Getenv("MINIO_WORM"), "on")
 }