readiness returns error quickly if any of the set is down (#9662)

This PR adds a new configuration parameter which allows readiness
check to respond within 10secs, this can be reduced to a lower value
if necessary using 

```
mc admin config set api ready_deadline=5s
```

 or

```
export MINIO_API_READY_DEADLINE=5s
```

cmd/globals.go

@@ -275,6 +275,9 @@ var (
 
 	// If writes to FS backend should be O_SYNC.
 	globalFSOSync bool
+
+	// Deadline by which /minio/health/ready should respond.
+	globalReadyDeadline time.Duration
 	// Add new variable global values here.
 )