Add a Slack notification

Created by: LinuxSuRen

What type of PR is this? /kind cleanup

What this PR does / why we need it:

Which issue(s) this PR fixes: None

Special notes for reviewers: Please add a secret into GitHub settings with SLACK_WEBHOOK_URL. Here is a tutorial about how to add a Slack webhook.

Additional documentation, usage docs, etc.: None.

This should be part of #1386