use nslookup in the init container

[kuizhiqing]

Background

To obtain the IP addresses of all pods in the same job, the paddle-operator uses an init container to perform a barrier operation. This makes it possible to collect the IP addresses of all pods and store them in a configmap that every pod can access.

Motivation

The recent implementation relied on the restclient to execute remote commands, which can sometimes have issues. Therefore, we replaced this mechanism with the nslookup tool to perform the barrier operation. This is expected to be more reliable.

Acknowledge

Thank you to @lfeng-nl for testing the work and fixing the issue with the following modifications:

• The svc may not work without setting publishNotReadyAddresses=True.
• alpine:3 does not work while alpine:3.10 works fine.