support import an existing clusters into the controller

[bbqccx]

I hope to support import an existing clusters into the controller to prevent issues such as accidentally deleting configurations in the controller or etcd crashes. With this feature, high availability can be quickly restored when the kvrocks cluster itself is not experiencing any abnormalities。

now 0.2 verion，if we create an existing kvrocks-cluster into controller will result in an error message: "Error: node(xx:8011) is already in cluster mode with version: 11."