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/s3-peer-client.go

@@ -48,7 +48,7 @@ func makeS3Peers(endpoints EndpointList) (s3PeerList s3Peers) {
 	})
 
 	hostSet := set.CreateStringSet(localAddr)
-	cred := serverConfig.GetCredential()
+	cred := globalServerConfig.GetCredential()
 	serviceEndpoint := path.Join(minioReservedBucketPath, s3Path)
 	for _, host := range GetRemotePeers(endpoints) {
 		if hostSet.Contains(host) {