Automatic merge from submit-queue Provide flags to use etcd3 backed storage ref: #24405 What's in this PR? - Add a new flag "storage-backend" to choose "etcd2" or "etcd3". By default (i.e. empty), it's "etcd2". - Take out etcd config code into a standalone package and let it create etcd2 or etcd3 storage backend given user input.