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/config/api/help.go

@@ -33,5 +33,11 @@ var (
 			Optional:    true,
 			Type:        "duration",
 		},
+		config.HelpKV{
+			Key:         apiReadyDeadline,
+			Description: `set the deadline for health check API /minio/health/ready e.g. "1m"`,
+			Optional:    true,
+			Type:        "duration",
+		},
 	}
 )