Automation: Add option to disable slack notification

[yonasberhe23]

Summary

Fixes #rancher/qa-tasks#2136

Added support to disable Slack notifications via the SLACK_NOTIFICATION environment variable (set to false in jenkins-job-builder/qa-ui-tests.yml). This enables debugging and testing of Jenkins pipeline issues without sending notifications.

Associated JJB pr: https://github.com/rancherlabs/jenkins-job-builder/pull/980

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