Fix signal handling for non-leader processes

[antonipp]

Description

This PR fixes a small issue: when Leader Election is enabled, all non-leader processes are waiting on:
https://github.com/GoogleCloudPlatform/spark-on-k8s-operator/blob/c261df66a00635509f7d8cb2a7fba4c602c9228e/main.go#L222

However, this means that they are never getting to this signal-handling logic below:
https://github.com/GoogleCloudPlatform/spark-on-k8s-operator/blob/c261df66a00635509f7d8cb2a7fba4c602c9228e/main.go#L234-L238

This results in non-leader processes not responding to SIGTERM or SIGINT, those signals are simply ignored. The only way to terminate those processes is a SIGKILL.

This PR makes sure that non-leader processes can be terminated with SIGTERM or SIGINT, just like the leader process.

Testing

Verified this behaviour in our Kubernetes environment:

• Before the PR, kill <PID> was ignored for non-leader processes but worked for the leader process
• After the PR, kill <PID> successfully terminates both leader and non-leader processes

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[liyinan926]

LGTM. Thanks.