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-current.go

@@ -371,6 +371,7 @@ func lookupConfigs(s config.Config) {
 	}
 
 	globalAPIThrottling.init(apiRequestsMax, apiConfig.APIRequestsDeadline)
+	globalReadyDeadline = apiConfig.APIReadyDeadline
 
 	if globalIsXL {
 		globalStorageClass, err = storageclass.LookupConfig(s[config.StorageClassSubSys][config.Default],