Simplify the steps to make changes to config.json (#5186)

This change introduces following simplified steps to follow 
during config migration.

```
 // Steps to move from version N to version N+1
 // 1. Add new struct serverConfigVN+1 in config-versions.go
 // 2. Set configCurrentVersion to "N+1"
 // 3. Set serverConfigCurrent to serverConfigVN+1
 // 4. Add new migration function (ex. func migrateVNToVN+1()) in config-migrate.go
 // 5. Call migrateVNToVN+1() from migrateConfig() in config-migrate.go
 // 6. Make changes in config-current_test.go for any test change
```

cmd/namespace-lock.go

@@ -53,7 +53,7 @@ type RWLockerSync interface {
 // Initialize distributed locking only in case of distributed setup.
 // Returns lock clients and the node index for the current server.
 func newDsyncNodes(endpoints EndpointList) (clnts []dsync.NetLocker, myNode int) {
-	cred := serverConfig.GetCredential()
+	cred := globalServerConfig.GetCredential()
 	clnts = make([]dsync.NetLocker, len(endpoints))
 	myNode = -1
 	for index, endpoint := range endpoints {